1. How do you stop bitcoin deposits? You can't stop people from sending transactions to their deposit wallets. You can maybe stop processing them, but imagine the outrage that'd cause

2. I don't think they can undo the already sent coins. If it really is because of using coinbase outputs, the transactions will confirm eventually (100 generations = ~17 hours?). Sending the money again would cause a double withdrawal.

3. Why? The faster it's fixed the less issues for them