You have excellent answer below.
The only other suggestion I can make is to have a face to face meeting between you, your Realtor, the sellers, and their Realtor, and figure out what can be done.
The best question to for your Realtor to ask the Listing agent is: "What can we do to move this transaction forward?".
You also might want to do an updated CMA and see if paying the extra property taxes, plus the purchase price, makes the transaction worth the price.
Ask your Realtor for a copy of the Short Sale approval.
The proces is most of the time like this:
An offer is submitted to the Mortgage holder with a HUD-1 Estimate (HUD 1 is the form that disclouse all the charges for seller and buyer, in this case the seller charges) . This form is review by the mortgage holder and some items will be approved (hopefully all of the charges) and some not. Most of the time those changes or changes on HUD-1 Estimate are told to the Listing Agent.
So, for more accurate information, request a copy of the short sale approval and the issue about property taxes being paid or not, should be there.