" ..But there is nothing inthe UCMJ that precludes civilians from seeing something wrong with the process, and addressing the situation."
That is correct, but he isn't a "civilian" he can not disrespect higher authority as a MILITARY OFFICER. He needed to retire or resign to become a civilian.
"So as a whistleblower, Scheller may or should have immunity."
He would as a civilian, as discussed above.