Sounds like Johns was an ass, but him calling up Fischer seems to have taken place after JL, when Johns was no longer in a position of authority over Fischer. Doesn’t really seem like Fischer has a case for Johns threatening his career during JL if that’s all he has, although Johns was clearly a petty Dick for calling him up if that’s true.
I don’t think he can be saved by sticking with Cavill. Spider-Man didn’t suffer at all from ditching the Garfieldverse. Snyder’s Superman is widely rejected and disliked. Battison has people excited, and Affleck was more well liked than Cavill, doubt people would care at all if Cavill was dropped. The X-Men have had three terrible movies back to back recently and they’re still coming back at some point. I get that you’re a /tv/ troll and a Snyder fanboy but honestly sticking with Cavill and Snyder is what’s going to kill the character, not dropping them.