I am not trying to be antagonistic at all, but even though your girlfriend is not "related" to you, I am still not sure the bank would look at the transaction as NOT being at arms length if you are in a relationship with the person buying the house and will moving back in after the short sale. Kinda touchy territory, so I would definitely speak to an RE attorney familiar with short sales and get their perspective of what the law and/or the banks may think.
I still think you should also speak to the bank about restructurig the loan if you have proof that the house is only worth $15,000 and you owe $98,000. Plus if the $15,000 is correct, I am not even sure the bank will eat a loss of $83,000 in a short sale. You would have to prove "hardship" to the bank and as far as I can see, if you are not behind in payments, haven't lost your job, or suffered any other financial hardship, the only hardship is that you don't want to keep the house.
Also just a thought, but since you won't be able to put your name on the title or the deed, what will happen if you and your girlfriend break up? She owns a house... and you? In addition to my previous comment about you credit rating getting screwed up for at least 5 year...all something to think about.
I understand your point about the finances, however, if the sole reason you are looking to sell the house is "strategic default", meaning you are not behind in payments, but don't want to "wait" 10 years till the value of the house comes up, that's a totally different story altogether. While not "illegal" , and IF the bank approved the short sale, it is IMHO, a bit unethical, and adding more fuel to the fire that has caused part of the housing collapse to begin with.