Rewriting the blockchain?
There's no need to consider the timestamps in order to know the difficulty. The difficulty is stored in the blocks.
This is a bit speculative but in the past such threats have been discussed, more along the lines of the hash function being hacked, and I think in such cases there will be a large consensus to hard fork. If the hypothetical threat you're describing is basically an adversary with infinite free power then most like PoW would have to be abandoned. Let's worry about that we get there. ;)