PoW miners will never be able to remove the whole PoS function and every block, but they could make the whole PoS system useless in order to maximize profits. I belive the rules you proposed are they only way besides CentralCheckpoints to solve the current doublespend problem.
PoW miners certainly won't maximize their profits by orphaning PoS. On the contrary - their mining revenue would be much lower if they decided to attempt orphaning PoS.