So, as an attacker with large stake, all I need to do is vote against valid transactions and just harvest all their stake? Neat.
Yes, that's how POS works, if you have more than half the stake you can do anything. But probably if you already have half the stake you are not interested in attacking anyone, but you'd like the value of the coin to go up 
No, it isn't. In regular PoS you can double spend if you own a large relative proportion of stake, but in your model, you can actually harvest all the stake from the entire rest of the network.