Are you buying this to live in, rent out long term, or resale after purchase??
Usually if all sellers on title have signed the purchase and sale and the bank has given back written acceptance approval of the short sale then you have a deal.
The issue might be getting a seller to sign closing docs that doesn't want to. You can try and make them perform with specific performance. You need to check and make sure there are no quirky laws in California protecting sellers from having to sell even with a written contract.
You might stand to gain some helpful knowledge by showing a real estate litigation attorney all your paperwork for review for a few hundred and they can give you realistic options and chances of being successful with your purchase if one seller fights it etc.
No legal advice
P.S. What is the reason the one seller is no contemplating backing out?? Try to find a solution to get all sellers back on track. The seller wanting to back out could have misinformed information and is making decisions based on bad advice they have received.