You can't attack 15 million coins of staked ETH with even 90 million un-staked ETH. You first have to stake the 90 million ETH. That's how the validators work. I am not sure you understand how it works. And if you attack the network you lose your ETH. Who's going to do that? George Soros?! lol!
Yes you can, In a Prof of wealth attack.
When 12% support POS and 88% support POW. You only need to hardfork, and remove the management.
This is how democracy works. (boris johnson)