First...if you have a signed contract from the seller "subject to approval by short sale lender(s)" you are not competing with other offers. The bank does not own the home and only has the right to approve or not approve an offer. So....not sure you need to worry about other offers.
That said.....even though the listing agent has been great, they still only have so much control over the situation..in fact the only control they have is to make sure they get everything to the bank as it's asked for....and then it's up to the lenders.
Depending on where things are in the process, and what the negotiator is telling the agent, they might have a good handle on how long things might take...but we've all been wrong on timelines before. There really is no guarantee....so you need to consider your personal situation to decide what to do.
Wish we could give you assurances, but there aren't any. Hope it works out for you....