Great questions. I think it's important that you ask if the agent has a back up agent - because the reality is that nobody is available 24/7. For example, some listings require that all showings be accompanied by the listing agent. Rather than miss out on a showing due to a conflict in schedules, the agent should have a back up agent who is well versed on your home and able to point out the best selling features of your home.
Typically, the current owner negotiates the deal with the potential buyer first and then the "accepted" offer is presented to the lender for consideration. In my experience, it's less frustrating to all parties if the short sale has already been "bank approved". You want to use a Realtor who has had short sale experience and who knows the right questions to ask the listing Realtor before an offer is even initiated. Best of luck.