The freq is recorded in the tower. I sincerely hope they gave the recordings to the cops or his lawyers so they can interview suspects and try find out who endangered those lives.
This is unclear from the transcripts, which seem to indicate that the request not to evacuate came from "Airport Command RF 2", i.e., one of the rescue fire fighters on the ground.
A Captain as PIC is in charge. But that does not mean that on Monday morning his decisions cannot
subsequently be evaluated by anyone (management, FAA, etc.) if those decisions may have needlessly endangered passengers and crew.
Personally I think the FAA will not get involved here.
From a strictly legal perspective, IMHO the Captain has a very tough case since Nevada is an "at will" employment state. But Allegiant made enough procedural mistakes in firing him that I think both parties will play for a settlement.
Btw the Captain's attorney was FAA's chief trial attorney in previous life.