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.
“It says in 4:6 that you are allowed to tell someone’s rebbi about his aveiros if that will help him, and you can even do so…”
You should have kept going. It says in 4:7 that you can speak LH on s/o who does like in the case of the OP -continously.