Considering the clues and evidence in Quranic verses and hadiths, Lady Marayam’s food would come straight from Allah (swt) and heaven without the mediation of any material thing or person from this world. What Islamic teachings tell us is that those in heaven will remain there forever, enjoying its blessings and pleasures which are always renewing, thus making them eternal and never-ending.
Nevertheless, whenever a blessing is used, it is actually ending, keeping this in mind, technically, one can't consider them eternal, despite the fact that they are always renewing and coming back after consumption.
On this basis, firstly, Lady Maryam’s use of heavenly food and fruit isn't in conflict with their eternity and secondly, there is a chance of her food coming from a barzakhi heaven, not the famous heaven that exists in the hereafter. Also, one must note that returning to this world from a barzakhi heaven is possible, and it is only when this world ends and the fate of everyone becomes clear that no one can come back to this world (because there will no longer be a world).
This question needs to be analyzed from different perspectives:
1- Was the food Prophet Zakariyya would see with Lady Maryam actually coming from heaven?
2- Is heavenly food necessarily eternal, or can it undergo change like other things in this material world?
3- Is it possible for people to return to this world from heaven?
We will address each of these questions respectively:
1- Regarding Lady Maryam’s food, the Quran says: “…Whenever Zakariyya visited her in the sanctuary, he would find provisions with her. He said," O Mary, from where does this come for you?" She said," It comes from Allah. Allah provides whomever He wishes without any reckoning”.
The verse doesn’t mention what type of food it was or where it would come from, but there are numerous hadiths in hadith sources that disclose that the food was fruits out of season that would become present next to her sanctuary out of divine decree. It isn't surprising for Allah (swt) to receive a righteous servant in such a manner, but some commentators of the Quran (such as the author of the Al-Minar tafsir) believe that what is meant by “provisions” is the normal worldly food that everyone uses, because Ibn Jurair has been narrated saying that when Bani Israel were suffering from famine and Prophet Zakariyya could no longer provide for Lady Maryam, several people cast lots in order to choose one who would take care of her, and the lot fell upon a merchant who accepted to provide food for her complimentarily. Whenever Zakariyya would come to Lady Maryam in her sanctuary he would be surprised at how good a food she had in such harsh circumstances and she would reply: “This is from God”, meaning that Allah (swt) has made such a service at such a harsh time cherished for a righteous individual and he willingly provides for me.
This explanation neither reads with the context clues the verse carries nor with what hadiths related to this verse tell us! A hadith by Imam Baqir (as) narrated by Tafsir Ayyashi says: “The prophet (pbuh) came to Lady Fatimah’s (as) home after days of no food being there, all of a sudden he saw some food and asked: “Where did this come from?! Lady Fatimah (as) answered: “It is from Allah (swt); He provides for whomever He wills without reckoning.” The prophet (pbuh) said that this incident was like that of Prophet Zakariyya’s when he came to Lady Maryam’s sanctuary and found a special food there and asked her: “O Maryam! Where is this food from?!” and she answered: “It is from Allah (swt)” (this shows that both incidents were similar, and that just like how Lady Fatimah had received from Allah (swt), Lady Maryam had also received from Him).
As for why “provisions” is referring to heavenly food, there are context clues that specify such, because firstly, the word “رزق” (provisions) has been used in its nakirah (indefinite) form, signifying that this food was something special and different that Zakariyya wasn’t familiar with, and secondly, Lady Maryam responds that it is from Allah (swt). Third of all, Zakariyya’s astonishment and excitement and asking Allah (swt) for a son in which the ensuing verse mentions, is also proof for our claim (because he was old and might have lost hope for having a son and successor, but when he saw how Allah (swt) directly had provided for her, he gained hope and asked Him for a son).
At the same time, the study of kalam or theology, has proven that Allah (swt) is the ultimate and one and only power and that creation, sustenance, bringing to life, taking of life, poverty, wealth, glory, abasement, health and sickness, etc., are all at His hands, nevertheless, they usually take place through their natural causes (both causes that have to do with our choice and ones that don’t) and sometimes not, such as the creation of Prophet Adam (pbuh) and Hawwa (Eve), etc. The prophets’ miracles are also good examples for such, also, the provisions Lady Maryam would receive were through causes other than natural ones.”
2- As for the second part of your question, it is a widely accepted law that substance and energy never cease to exist (are eternal), but continuously change into each other in different forms. Food that we consume in this world goes through the same process; some of it is changed to energy and some of it to substance, while the rest is excreted. Nevertheless, there is no reason to say that this is the only way for changing substance, and at the least it can be said that it is logically possible for there to be other ways for food not to bear any substances that will end up as waste to be excreted, or for food to be completely changed to energy, leaving behind no substance at all to be excreted as waste.
As for heaven and its never-ending blessings and pleasures and the eternal life there that the Quran and hadiths stress one, it doesn’t mean that even the food that they eat from is eternal; as a matter of fact, there are reasons that suggest the opposite.
Zayd ibn Arqam narrates that a man from the people of the book came to the prophet (pbuh) and said: “You believe that those in heaven eat and drink.” The prophet (pbuh) replied: “Yes…”. The man then asked if they eat and drink, then how do they relieve themselves of the waste of what they consume? The prophet (pbuh) replied: “This substance (that needs to be excreted) is excreted as a very aromatic sweat, smelling like musk…”.
In other words, blessings in heaven don’t vanish and cease to exist, but undergo change and transformation.
Keeping what was said and also Allah's (swt) never-ending power in mind, no problems are entailed by granting certain chosen servants of Allah (swt) heavenly provisions and it is totally possible.
3- The third part of your question about the return of those in heaven to this world: There are at least two meanings for heaven and hell in Islamic sources and scriptures:
a) The heaven and hell that commence at the death of an individual in the barzakh. We read in a hadith that each person’s grave is either a garden of paradise or a pit of hell for him (depending on him being good or bad in this world). Considering the different cases of coming back to life mentioned in the quran, it is possible for people to return from such heavens and hells (in the barzakh).
b) The heaven and hell that people will go to after ‘the trumpet’ has been blown in and all have been judged for their deeds. Leaving this heaven, according to what the Quran directly tells us, is impossible, and in reality, there will no longer be a world to return to in the first place.
For further information, see:
1- The existence of heaven and hell in the present time, Question 101 (website: 1798).
2- The true essence of heaven and hell, Question 1945 (website: 1948).
3- The blessings of heaven not being tiring, Question 1823 (website: 1977).
 Muhammad Baqir Majlisi, Biharul-Anwar, vol. 14, pg. 195.
 Abd Ali ibn Jumah Arusi Huwayzi, Nurul-Thaqalain, vol. 1, pg. 333.
 Tafsir Nemouneh, vol. 2, pg. 530; Al-Mizan, vol. 3, pp. 273-274.
 Tafsir Nemouneh , vol. 2, pg. 530.
 Biharul-Anwar, vol. 8, pg. 149, hadith 82.
 Biharul-Anwar, vol. 6, pg. 214.