This relatively detailed question must be addressed from different perspectives:

1- How is the eternal nature of Islamic laws explained?

2- Do we perceive the cease of execution of a certain law for a period of time because of extenuating circumstances or for something more important as breaking the law?

3- How related are the things you mentioned in corroboration of your claims with what you were asking about?

Now the answers to each of these questions in the same order mentioned above:

1- Regarding the first question one must say that generally divine religions are comprised of two parts, the rules that have an eternal nature and those that are unstable and change. The rules that are permanent relate to the stable aspects of humans life. But these religions also have paid attention to the changing aspects of man's life through legislating tenets which change and vary depending on the time, place, economic, cultural and social circumstances.

What is meant by the eternal nature of the Islamic law is that the permanent and unchanging rules are so stable they will stay remained unchanged forever, but the methods used to carry them out have the flexibility to change in different situations. The change of these tenets which can also be seen during the time of the prophet himself does not entail the change of the general and eternal rulings, for more explanation you can refer to the seventeenth volume of Tafsir Nemouneh, page 341 which is about the final and seal of religions and to Question 781 of this website which relates to the comprehensiveness of religion.

2- The fact that the general rules are eternal in Islam does not mean we have no choice but to be fundamentalists and carry out religious law while ignoring the circumstances and situation we are in! In other words the eternal aspect of Islamic law is somewhat intertwined with the changing aspects of human's life such that the two cannot be separated from each other. This is the reason why issues like secondary rules, the clash of the important and more important, ezterar (extenuating circumstances), ekrah (being forced) have all been discussed in Islamic jurisprudence. [1]

When examining the Quran we come across cases where even denying the oneness of Allah is allowed due to a certain situation. [2] When "Ammar Yaser", one of the greatest companions of the prophet, denied the oneness of Allah (swt) to save his life, not only was he not blamed and criticized buy the Quran, but the prophet and the Imams all supported and backed his act! [3] If denying the oneness of Allah which is the most fundamental belief in Islam is allowed in certain situations then obviously putting aside some of the rules that are of less importance in such situations is allowed too. Let's say stealing from others is a considered a sin in Islam, but what if Islam said that if someone was so hungry and needy that he would die without stealing he won't be punished for it, does this mean that we are denying the fact that stealing is a sin?!

What if we stop carrying out some of the rules for a specific period of time because of the potential global dangers that threat the lives of Muslims? Does this mean we are putting this law aside? Obviously the answer is no. When referring to the books of jurisprudence we find a notable difference between "Ilgha" (nullification) and "Ta’ligh" (suspension of the law). "Ilagha" means to put a certain rule away completely, but "Taligh" means to cease the execution of a ruling for a period of time due to certain circumstances. If one pays close attention, he will see that in many cases, what is actually happening is that the execution of a law is being suspended, not nullified!

3- As for the examples that were mentioned in the question as cases where Islamic law has been changed, we will analyze them one by one to see what has really taken place:

a) The delegation of the right of divorce by men to women does not entail the change of the law, because this right still lies in the hands of the man and it is only when a woman is facing serious circumstances that the man has given her the right to carry out the divorce on his behalf. In this regard there are a few points that we must pay attention to:

First:  When someone delegates a certain right he still has that right, so when a man gives this right to his wife he does not lose his right, because delegation does not take place if one does not have a right to grant to others. [4]

Second: She only has a right of divorce when she is facing hard circumstances and having a hard time with her husband and continuing to live with him becomes very harsh and intolerable.

Third: Taking the second point into consideration it is obvious that even if the right of divorce was not passed down to her and she was being bothered and oppressed by her husband she could always refer to a judge and get divorced, the fact that Islam has granted men with the right of divorce in the first place does not mean Islam will let men keep women in a hard and difficult situation where their rights like receiving "Nafaqah" are not respected. [5] So women actually possess this right from the beginning and what is really happening when the two agree that the woman have the right to execute the divorce is to ease the process without any need to go to a judge and other time-consuming stages.

b) Regarding the issue of Diyah that was pointed out in the question, there are two points that seem to have been overlooked:

1- In contrast to what you have stated in your question, the law of the diyah of the People of the Book is not mentioned in the Quran and it is through the hadith and tradition that we understand the rulings in this regard.

2- There is no mention in our hadiths that the Diyah of the 'People of The Book' is half of that of Muslims, the only thing we have about being half, is the diyah of women compared to men. The amount of the Diyah of the People of the Book is mentioned as being "800 Dirhams" in most books of jurisprudence. [6]

Now, keeping these two points in mind, we will analyze this matter:

Islamic scholars in different countries have had different viewpoints regarding the amount of Diyah for Non Muslims, and most Shia scholars have accepted the amount of "800 dirhams" [7] , this has also been accepted today, but due to the agreements that we have accepted and the special international circumstances that were are living in this law has been changed through a "secondary ruling".

The fact that their Diyah is equal to the Diyah of  Muslims is against primary Islamic law and that is why Council of Guardians (who bears the responsibility of making sure the country’s laws are in accordance with Islamic law) has not accepted such a law, but it is because of a secondary ruling and that it sees it to the benefit of the country today that the Expediency Discernment Council  has decided to take on the new law. It is easy to forget that what the Expediency Discernment Council  has done is not breaking the Islamic law, but rather the secondary ruling is taking precedence over the primary ruling due to its importance. When we look back on the history of the prophet we find that such a thing was done at his time too. For example in the treaty of 'Hudaibiyya' he surrendered one of his followers to the polytheists because of an agreement he had arranged/signed with them. [8] This was done even though the primary Islamic law does not allow the turning over of a Muslim prisoner to the Non Muslims, but the prophet did so according to these secondary rulings and at that time some Muslims who didn't understand these rulings opposed his opinion.

Therefore regarding the issue of Diyah the Islamic law has not been changed but its execution has been temporarily suspended due to the situation we are in; this doesn’t mean that Allah (swt) wasn’t aware of the fact that certain situations will come up that call for other rulings, rather the general rulings in Islam are set for usual situations and when special situations come up, we have the secondary rulings that come into play.

c) The next part of the question had to do with the three issues of Heritage, Will and Bazl (giving money or other belongings to others), and all three of them were mixed, so before anything else we will clearly define these three concepts:

Bazl: Every healthy and sane individual is in charge of his wealth and belongings and determines how to use them, for example he can use them to fulfill his or his family's needs, he can give them in charity for the needy, or he can grant it to others as a gift which is called Bazl, therefore no one has the right to stop him from doing so, though this right of his is limited when he has a disease that ends in his death.

Will: To will means for a person that is going to die to command his family to do a set of things that may not even relate to financial affairs. According to the Islamic law one can spend his money as he likes but can only determine what happens to one third of it [9] after his death and the rest belongs to the inheritors.

Heritage: The first thing done to the money one leaves behind after he passes away is that his debts are paid from that money, in the next step the inheritors should act according to his will regarding one third of the money, then the rest is to be divided between the male and female inheritors in a ratio of 2 to 1. [10]

Therefore the heritage of one who passes away is only considered after his debts are paid and his will is acted upon and the amount of heritage that family members will receive does not change. Nevertheless one can financially support some family members more than others while alive, or can give them more money through his will (that only has to do with one third of his money), but as we explained above he cannot will regarding to two thirds of his money and all of it will be given to his inheritors. After all inheritors have received their share they own that money and can decide what to with it, they can keep it for themselves or they can grant it to other people like their brothers and sisters, and granting their money has nothing to do with the ratio in which the heritage is given to the inheritors, for this giving is done after they have received their shares.

If you have questions regarding the difference between men's and women's inheritance you can refer to this site to find your answers: Question 2207 can especially be helpful.

d) Although the mixing of men and women is not allowed according to the traditions and the Sira (lifestyle) of the prophet and his household and the Holy Quran that calls both genders to modesty in various verses [11] , but not all kinds of mixing is forbidden, for example men and women can coexist in society if being together does not cause sins. Even during the time of the prophet men and women would work together for the progress of the society. Verse 32 of Surah Ahzab can be good proof for such a assertion, the verse says: "Wives of the Prophet, you are not as other women, If you are god fearing, be not abject in your speech, so that he in whose heart is sickness may be lustful; but speak honorable words" [12] We must pay attention that the verse does not forbid speaking to anyone who is not mahram, but it is forbidding the kind of speech that may entail the committing of sin, it goes the same about the physical mixing between men and women, meaning that if the mixing will lead to the committing of sins, then it is Haram.

As for the mixing of men and women in Hajj, since there is a lack of sufficient space there for pilgrims, in order to solve the problem of the mixing of men and women, different solutions are possible:

First: In order to prevent the mix of women and men the practice of this ritual should be stopped! Obviously such an idea would be greatly challenged because of the important role Hajj plays in Islam, and its social effect and influence on the Muslim community through out the world. This ritual is so critical that Islam says in the case of the people not wanting to go, the leaders of Muslim countries are responsible to pay instead of them and use force to send them to Hajj if they do not cooperate. [13]

Therefore no one can ask for Hajj to be ceased just because their may be a mixing of men and women.

Second: The next solution would be to prevent one of the two genders (for example women) from performing the pilgrimage, which does not make sense either, because pilgrimage is just as much essential for men as it is for women, to the extent that women must make the trip to Mecca even without their husband's permission if the Hajj has become wajib upon them. [14]

Third: To separate the men and women in a way that each group would do the ritual in a special part of the mosque so that they won't be forced to pass by each other or bump into each other while performing tawaf, this is a good solution and it has been put to practice in other places during hajj like the tents in Mina and Arafat. We hope that the authorities of Hajj and Muslim scholars will work hand in hand to take care of this problem.

But until this problem is solved Muslims should be careful not to be tricked by Satan in committing sins during Tawaf, obviously participating in Hajj can not be considered permission to touch one of the opposite sex, therefore what was a sin before Hajj is still a sin during Hajj and Tawaf. Some pilgrims assume that men and women are considered brothers and sisters during hajj, but the truth of the matter is that none of the rulings regarding men and women change during Hajj. For example there is a Hadith which says a woman was doing tawaf and her hand and forearm were exposed for a moment, a man that was doing tawaf behind her put his hands on hers! All of a sudden and in miraculous way their hands stuck to each other, eventually all of the other pilgrims stopped and stared at the two of them. The governor of Mecca was informed and the ulema of the city asked the governor to cut off the hand of the man. Finally they explained what had taken place to Imam Hossein (that was in Mecca at the time also performing the pilgrimage).  He then raised his hands and prayed and supplicated for a long time and walked to the woman and moved the man's hand away from the woman's, the governor asked: "Should I command my soldiers to cut off the man's hand?" The Imam told them not to (and thought that Allah's (swt) punishment was enough for him)" [15] .

Therefore none of what was mentioned in the question were actually a change of the Islamic law. We must keep in mind that the flexibility of the rulings and tenets in Islam is not only in harmony with their eternal nature but rather makes it possible to carry them out in different situations without causing changes in the rulings.

Related question: Question 6967, Revocation of capital punishment and banning of torture in Iran (website: 7091).