Unfortunately, I cannot answer the second part of your question, but I will attempt to answer the first. Regarding Abraham and the binding of Isaac, child sacrifice was the norm in ancient pagan culture.[1] People felt this was a good way to show of G-d. For example, there is a possible link to Greek myth, though I am convinced it was independently derived by the Greeks.
In Greek myth, King Agamemnon was commanded to sacrifice his daughter Iphigenia by the goddess Artemis. One version has the king succeed and is later killed by his son, Perseus, yet another version tells that the hero Odysseus, in angle-like fashion, shouted to Agamemnon: “The goddess wants the deer, not the maiden.” Thus, Iphigenia was set loose and a deer was sacrificed in her place much like the ram in Genesis 22. In fact, this version closely resembles the Akedah story, the binding of Isaac.
So when G-d commanded Abraham to sacrifice his child, he would do so because you don't argue with G-d. Now Abraham must have known that the Torah prohibits child sacrifice and so he must have felt perplexed at this command, possibly realizing that G-d would not enforce it anyway. In any event, a Midrash claims that an angel approached Abraham, yelling at him to stop, however, Abraham declared that he will obey G-d's command and again, the angel shouted, "do not harm the boy, do nothing to him."[2] Thus, the patriarch Abraham repelled the brute force of his knife and set Isaac free, offering a ram in his place.[3]
If the Midrash is telling a true event, then it is possible that Abraham was really tricking G-d, that he never intended to kill Isaac. But if G-d is all-knowing and all-powerful, He would have known Abraham's intent and never sent an angel to warn him. In that case, it would make more sense to say the story as a whole is a parable and is no more than daytime thinking. If it was an internal struggle, Abrahams might have felt that he should sacrifice what's most important to him, his son. Many ancient cultures practiced this ritual as seen above, the way of the pagans. In due course, Abraham reflected and decided that G-d is not cruel and would never demand the death of his son.[4]
The rabbinical statements in the Talmud say that Abraham observed all the Torah enactments before the Torah was given at Sinai.[5] Accordingly, G-d gave the option of following the Torah to the patriarchs, though they were not obliged to do so. It is possible that the many seemingly contradictions between the Torah and rabbinic enactments weren't that the rabbis made up the Oral Law at a later date but rather that the patriarchs chose not to obey all the laws since they were only optional. Yet others feel that the hyperbolic sermons were designed to highlight the importance of Torah observance for if the Torah were revealed to them the Torah would have stressed the importance in different ways - yet Abraham was righteous in the eyes of the L-rd G-d for “Abraham listened to my voice and kept my charge, commandments, statutes, and laws.”[6]
Genesis 26:5 states that G-d said that Abraham "listened to my voice, and kept my charge, my commandments, my statutes, and my laws, the Talmud and Midrash Genesis Rabba comments that Abraham observed all the biblical and rabbinical commands. The rabbis here are emphasizing not only the importance of the Oral Torah (or, Oral Law), but that the Torah and rabbinic laws are part of nature, natural laws, something intelligent man such as Abraham could deduce from nature. Indeed the Rambam quotes a midrash which says that Abraham discovered G-d by studying the heavens, natural law. Additionally, G-d revealed to Abraham and gave the option of observing the mitzvot and all the laws of Judaism to the patriarchs who could choose to observe them fully, partially, or none at all; it was optional. They did it to show love of G-d.
To answer part two of your question on whether a prophet can temporarily nullify a biblical enactment please see here How to reconcile the concept of hora'at sha' ah in regards to a legitimate prophet?
[1] For example, the king of Moab sacrificed his firstborn in a burnt offering.
[2] See Genesis 22:12. The Midrash Genesis Rabbah explains that although Abraham was being submissive to G-d's command he would never maim the child on his own.
[3] Talmud, Sanhedrin 89b says that a prophet can suspend a law, apparently even if its murder as in the case of Genesis 22.
[4] Guide 3.24.
[5] Talmud (Tractate Yoma 28b).
[6] [Genesis 26:5].
