PBA -“Isn’t this identical to the story of Miriam, where both miriam and aharon knew what had happened, and it was still lashon hara to talk about it it in a derogatory manner”
No, of course not. Moshe Rabbeynu never did anything wrong. Miriam had absolutely no Heter to speak about him.
I don’t think it is relevant to lashon hara whether what the person did is actually wrong, unless the action takes the person out of klal amisecha, which I don’t think is relevant here.
That answers your second point as well.