Wolf — The Meforshim speak about this issue concerning aunt-nephew marriages. While it is true that everyone has a din of Ben Noach prior to Matan Torah, there is a differentiation between mother’s side vs. father’s side.
Yekke2 — You are correct concerning Moshe and Aharon prior to the Mishkan. But if we look at Rashi in Bereishit 14, 20 when Avraham gave Maaser to MalkiTzedek, it’s mefurash that MalkiTzedek was a Cohen which suggests that the entire lineage from Shem (Shem=MalkiTzedek) onwards through to Levi (and Amram) were Cohanim.
WIY — Concerning Eldad and Meidad: The interpretation you listed is one of three explanations. Rabbenu BeChai brings down that they were 2 unrelatd individuals (not even brothers to themselves) from different Shvatim. And Da’at Zekenim MiBaalei Tosafot suggest that they were brothers to Aharon from Amram’s side, not Yocheved’s side.
All of the above, compels me to humbly suggest that Amram may have had 2 wives (as did Yaakov and Elkanah, etc…) and that Amram did not “divorce” Yocheved but merely separated from her.