Thus any large escrow service would be a threat to the network, in addition to large miners.
Yes, they can, but technically it will be suicide for them. Anyway, it's possible to prevent such attacks by implementing another REORGANIZE algo.
why reorganize? it is still harder and extremally expensive to 51% a PoS blockchain, than a PoW only one.
beauty of PoS concept is that atacker to be succesfuf has to attack himself.