I am not an expert in BitShares no. However, I know that OpenLedger have recommended pulling your Bitcoin from the Exchange if you want Bitcoin Cash, since that will not be available to you, if you leave Bitcoin on the DEX. However, I think all transfers of Bitcoin will be locked tomorrow 11 PM GMT on OpenLedger (and I suppose therefore the whole BitShares Exchange). The asset OpenBTC is not affected. OpenLedger have a news post on it here:
https://blog.openledger.info/2017/07/28/btc-bcc-and-hard-forks-oh-my/
To answer your questions, I don't hold Bitcoin on BitShares, so I don't know. What I do know, is that you're better off not having Bitcoin on any exchange. It's only really yours if you hold them in a wallet you own.
Again, I am absolutely no expert, so others will have to correct me on the above. This is just my understanding.
Your answer was all amazing. Thank you for the effort!