I'm curious if there's an argument to be made that Egypt must reopen the Rafah crossing on their side (to allow humanitarian aid to flow into Gaza), even if they don't like who's controlling the other side of the crossing presently, i.e. the IDF.

On a quick search, the Geneva Convention doesn't seem to cover this case because Egypt is not an "occupying power", so has no obvious obligation towards the population of Gaza in that regard. (Also ICRC rule 55 is limited to "the parties of the conflict". Where 'conflict' means a military one, not just a diplomatic dispute, like Egypt is having, at the moment.) But maybe there is some other argument to be made?