The theory that there will be two (or more) sorts of coins is quite old. It's also possible that the price for the different classes of coins will be different. Anyway, I take it as chance to actually build a parallel structures with it's own money.

I don't believe that 51% attack is feasible for anyone. Just look at the hash reate right now. How many cards you need to buy (and/or manufacture)!. Not to mention that you need a place/cooling/power. That's (hundreds of) $ billons. You can't hide such as operation. Someone will notice that. And in that case all others have to do is to add let's say another 5% to stop them.

Also, who is them? Do you think that let's say China will cooperate with US gov. on this plan?