Was he even making money off the casino besides his salary?
He wasn't on the books. Phillip Green was the front man. Ace unofficially ran it so they could circumvent the Nevada laws. He would have been paid indirectly by the mob and they would have paid handsomely considering he doubled profits giving them far, far more to skim off.
It wasn't his casino, he just ran it. So when Nicky is giving him trouble why didn't he just talk to the mob back home and say "send someone else because this fucking guy is bringing too much federal attention on us and he's gonna fuck everything up"
He literally says that to Andy Stone and Stone tells him they're not sending Nicky anywhere. Remember Nicky is making a lot of money for them too and he now had a complete foothold on organised crime in Las Vegas. They send him away they risk losing control there.
he made his money from gambling, so being fired and having corporations take over the casinos would barely have impacted his earnings at all.
It would have impacted his earning significantly and he'd also no longer have people lining up to ask for favours and giving him access to benefits and societal perks in return.
Did you watch the movie?