This is probably the best way to handle things. Once BU activates, the market will be able to decide which chain will emerge as the "winner" as the price will give the miners an incentive to mine on the higher priced coin.

In regards to what "blockstream coin" and "bitcoin unlimited" are called on the exchanges, I really do not think this matters, even if exchanges call either of them something other than Bitcoin for a long time. Although it is very likely that one chain will very quickly die due to the incentives associated with mining and the time it takes for the difficulty to recalculate.

I agree that there should be something to prevent replay attacks on the protocol level, both for the exchanges' sake, and for the protection of the end user. Although as mentioned above, one chain will likely die, and if this happens, replay attacks will be more or less a non-issue.