I would never advise someone to simply walk away ... at a bare minimum, I believe a homeowner needs to make a solid effort at a short sale.
Your questions and concerns need to be addressed with your agent, and a game plan identified.
Good luck to you.
Most real estate contracts allow FAX signatures, I suggest speaking with your agent to see if that's allowed in your situation.
You will still need to prepare all of the paper work and participate in the short sale process to some extent. Sometimes the first mortgage holder offers the second some money to get them to settle.
Also, should something change an you come back to the US sooner than you anticipate, the short sale will have less impact on your credit than the foreclosure.
I think it would be wise to hire an attorney if you can to handle the transaction for you in your absence. If you cannot afford to do that please ask a trusted family member to act as your power of attorney. Keep in mind, that the mortgage lender (s) may object to you not being personally involved in any negotiations.
None of us here can advise you as to your best course of action regarding walking away versus negotiating with your mortgage lenders. Only you know how you'll handle the emotional aspects and financial aspects of walking away from your debt owed to the mortgage lenders.
Good luck to you and I hope your new job works out well for you.