1) Tosfos (Zevachim 102) quotes a Midrash that Aharon begged Moshe to have mercy on Miriam because he worried that were she to become a metzora, he would not be able to be metaheir her. Since a kohen cannot pasken on the negaim of a relative and there were no other non-related kohanim to turn to, Miriam would be out of luck. Tosfos asks: if Aharon couldn’t pasken on the nega, then he couldn’t be metamei Miriam either, so what was he worried about?!
The Netziv answers that Aharon was in fact worried about exactly that – that he would be unable to do anything, not be metamei or metaheir. If nothing happened, then Miriam would have to live without any absolution for her sin. Not having a kapparah is worse than suffering the pain of being a metzorah for a week and earning forgiveness.
2) The Torah tells us that Hashem appeared “pisom,” suddenly, to Moshe, Aharon, and Miriam. Netziv explains that the intent was to shock. We understand why Hashem wanted to shock Miriam and Aharon, but why shock Moshe? I would have said that Hashem wanted to demonstrate the difference between Moshe and his siblings. Moshe was so attuned to G-d’s presence that he might not startled by the G-d’s appearance in the name way his brother and sister were, proving the superiority of his level of nevuah. Netziv, however, answers that when Hashem first appeared to Moshe way back in parshas Shmos, he turned Moshe’s staff into a snake and Moshe drew back in fear (4:3). Considering that he was in G-d’s presence, Moshe had nothing to fear, but the reaction was spontaneous and instinctive – he didn’t pause to think. As a punishment for that instinctive withdrawal, Hashem sprung his presence on Moshe now and shocked him.
I don’t get it – why would Hashem mete out punishment now for something that occurred much earlier, before Moshe’s shlichus even started?
Perhaps the answer is that the fact that Moshe was being punished for such an infraction was itself a testimony to his greatness. An instinctive reaction would never count against a lesser person and they would never be blamed. It could only count against a person on the level of a Moshe Rabeinu. Therefore, precisely in this context, where the Torah comes to testify to Moshe’s greatness, Moshe receives a punishment for what until now had been overlooked.