I think it's still considered a purchase money loan, because even though you're probably on title because of the AITD contract, you were never the primary borrower.
The big question is, where is the home value now in relation to what is owed by the primary borrower / 'previous owner'. If the current loan amount is less than what you can get it appraised for, a short sale would be the only way you could get the loan in your name.
Let me know if you have any questions. I won't respond to this thread so email me directly. Alternatively you could call my lender if you'd like to talk to her about putting the loan in your name. She works weekends: 562-552-1191 or ASpellman@StratisFinancial.com