You are right. This is something what could be a real problem. If the same chain hopping happens like with Bitcoin Cash, you can run in exactly your described situation.
I guess the only secure way would be just to wait until both chains have a stable hash rate to minimize this risk.
Nevertheless I also would prefere that the Segwit2x devs implement a replay protection. Even if the timelock trick would work 100%, with the current clients it is too complicated to do for the average user.
It is not even about the stable hashrate. These rapid increases in the mempool come out of nowhere. If they all of a sudden come out on the logger chain, you are stuck and the other chain will catch up by the time your transaction is confirmed in the longer chain. So this problem isn't just going to go away in few weeks, it is there forever. These two chains can't safely coexist. Using one means that you completely ignore the other as it doesn't exist.