The above question has been propounded in two parts one of which has been dealt with in our previous answer to similar questions. The other part consists of two main points that need to be studied here. One, the duty of mankind is to believe in the divine prophets and not to differentiate between them in terms of their righteousness and their being the prophets of God. The second point to focus upon is the rank and virtue of the prophets in comparison with each other.
Given that human beings have been created free to fulfill their religious obligations and duties and that different individuals have different capacities, it can therefore be said that any variance in rank and virtue (even amongst the Infallibles) is something that does not affect or impair our beliefs.
First of all, we need to remind you that the above question has been propounded in two parts one of which has been dealt with in our previous answer to a similar question which is available on the website.
The other part of the question revolves around two main points which we shall discuss and deal with here:
1. Man's duty to believe in the past prophets and in their books: When it comes to this point, the Quran explicitly states that the believers should believe in all of the prophets (a) without making any distinction between any of them. In fact, when we believe in a prophet, we should believe in whatever he says. And we know for sure that one of the doctrines of our Prophet, peace be upon him and his family, is to admit and confirm the previous prophets. Hence, it is the duty of Muslims to accept those prophets and have faith in them. The verses which you have referred to in your message and which will be mentioned below denote the same meaning:
A) " Say: We believe in Allah and (in) that which had been revealed to us, and (in) that which was revealed to Ibrahim and Ismail and Ishaq and Yaqoub and the tribes, and (in) that which was given to Musa and Isa, and (in) that which was given to the prophets from their Lord, we do not make any distinction between any of them, and to Him do we submit." 
B) "The Messenger believeth in what hath been revealed to him from his Lord, as do the men of faith. Each one (of them) believeth in Allah, His angels, His books, and His messengers. "We make no distinction (they say) between one and another of His messengers." And they say: "We hear, and we obey: (We seek) Thy forgiveness, our Lord, and to Thee is the end of all journeys." 
C) "Say: "We believe in Allah, and in what has been revealed to us and what was revealed to Abraham, Ismail, Isaac, Jacob, and the Tribes, and in (the Books) given to Moses, Jesus, and the prophets, from their Lord: We make no distinction between one and another among them, and to Him do we submit.." 
2. Superiority of some of the prophets over others : The superiority of some of the Prophets over others has something to do with the religious, divine and spiritual personality of the prophets themselves, not with our duty towards them.
Given that human beings have been created free to fulfill their religious obligations and duties and that different individuals have different capacities, it can therefore said that difference in their ranks and virtue is essential and self-evident. We cannot subscribe to something other than this. In this regard, the Quran also refers, in many verses, to these distinctions:
A) "Of those messengers, some of whom We have caused to excel others." 
B) "And we preferred some of the prophets above others." 
For information about the second part of the question, you can refer to the following indexes:
- "The Meaning of Infallibility No1737 (site: 1733)",
- Sunnis on the Infallibility of the Prophet (s) in areas other than revelation, No. 5071 (site: 5436)".
- "Infallibility is not Compulsory" 6918 (site: 7018)"
- The Non-Mandatoriness of Infallibility and the Infallibles' being as Role Models, No. 7156 (site; 7267)
- The Infallibility of the Imams, No. 13388 (site: 13374).
 - Al-Baqarah: 136:
" قُولُوا آمَنَّا بِاللَّهِ وَ ما أُنْزِلَ إِلَینا وَ ما أُنْزِلَ إِلى إِبْراهِیمَ وَ إِسْماعِیلَ وَ إِسْحاقَ وَ یعْقُوبَ وَ الْأَسْباطِ وَ ما أُوتِی مُوسى وَ عِیسى وَ ما أُوتِی النَّبِیونَ مِنْ رَبِّهِمْ لا نُفَرِّقُ بَینَ أَحَدٍ مِنْهُمْ.
 - Al-Baqarah: 285:
آمَنَ الرَّسُولُ بِما أُنْزِلَ إِلَیهِ مِنْ رَبِّهِ وَ الْمُؤْمِنُونَ کُلٌّ آمَنَ بِاللَّهِ وَ مَلائِکَتِهِ وَ کُتُبِهِ وَ رُسُلِهِ لا نُفَرِّقُ بَینَ أَحَدٍ مِنْ رُسُلِهِ وَ قالُوا سَمِعْنا وَ أَطَعْنا غُفْرانَکَ رَبَّنا وَ إِلَیکَ الْمَصیرُ.
 - Aal-e Imran: 84:
قُلْ آمَنَّا بِاللَّهِ وَ ما أُنْزِلَ عَلَیْنا وَ ما أُنْزِلَ عَلى إِبْراهیمَ وَ إِسْماعیلَ وَ إِسْحاقَ وَ یَعْقُوبَ وَ الْأَسْباطِ وَ ما أُوتِیَ مُوسى وَ عیسى وَ النَّبِیُّونَ مِنْ رَبِّهِمْ لا نُفَرِّقُ بَیْنَ أَحَدٍ مِنْهُمْ وَ نَحْنُ لَهُ مُسْلِمُون.
 - Al-Baqarah: 253 تِلْکَ الرُّسُلُ فَضَّلْنا بَعْضَهُم عَلى بَعْض.
 - Isra: 55, وَ لَقَدْ فَضَّلْنا بَعْضَ النَّبِیینَ عَلى بَعْضٍ.