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.
All they would have to do is make it useless for the purposes it should serve. Centralizes PoS would work but would also be pointless.
Examples:
1)All miners would benefit if most blocks are PoW and the few remaining (big) PoS are paying a fixed fee. Rich adresses would benefit from excluding small wallets from PoS-rewards. The other people don't matter.
2)Even if miners can't agree on a fixed fee, they could agree on a minimum size for a wallet to PoS. (Higher minimum->less PoS blocks ->more PoW->far more new coins generated->more profit). Benefits both miners and rich guys.
3)When several PoS blocks are competeing for one spot miners can decide who will win the spot. Combined with richguys spliting up their wallets they could together require a fixed fee for a PoS-spot. It doesn't matter for the miner on what block they mine ontop of.
All those possibilities would destroy the benefits of PoS and make it useless. That are just 3 obvious ones and it's likely that there is a non obvious but profitable one. The problem with your rules is that they can be easily exploided by a few for just their own benefit. Usually if that's possible it will also be exploided (examples:banks, govermets,... mankind)
Is cementing an option, or would that be really bad with our current distribution of hashpower (mostly centralized)? https://en.bitcoin.it/wiki/Proof_of_Stake#Cementing
Very very (very) bad idea ... NO.