Forks suffer from a problem known as transaction replaying - If you tried to spend coins on one fork, it would probably be spent on the other fork too, which makes it nearly impossible for exchanges to accept/trade both.
This is not true. In the event of a chain split, the exchanges would likely temporarily suspend deposits/withdrawals temporarily. Exchanges would then "split" some of their coins by sending a transaction to one address on one chain, then sending a transaction that depends on the same inputs to another address on the other chain. Once both transactions are confirmed, the exchanges may repeat this process in order to have a greater number of coins that are outright "split". After this happens, any withdrawal that the exchanges processes will be dependent on the split transaction(s) on each chain -- exchanges could use two inputs, one from the "combined/unsplit" chain, and one from a specific chain, and this transaction would only be valid on one chain.
Due to certain mining incentives, it is unlikely that very many new blocks will be found on the minority chain with the same type of PoW. If "Bitcoin Core" chain is to survive, then the Core dev team would most likely HF to a new PoW, which would mostly eliminate the risk of a "wipeout" and 51% type attacks on either chain. Until this happens, it is most likely that deposits and withdrawals will not be processed due to the high risk of monitory loss.
Exchanges would only trade in the original (core) fork until BU adds the necessary replay protections to their fork, and have said as much.
This is also not true. Exchanges will list coins that have sufficient demand by their customers. (see my above comment).