No one can "guarantee" the sale will close before they find another house.

I guarantee the cost of the hotel is higher than the rent they are paying, so while they may feel inconvenienced a bit, it is a real financial cost for you to do this for them and  (here is the important point you make to them) that you are not obligated to do, but are choosing to because you value them as tenants. 

well that's called a saber rattler letter.. big bluff 95% of the time guarantee attorney wont work on contingency so this lady / tenant would have to pay to sue you.. this stuff is handled in small claims usually.and your a long way from then winning a judgement.. if it was me I would just not respond.unless they actually sue you.. or if your really nervous about it send the 1500 back..you could talk to a PM probably for free that would know this off the top of their head .. short of that maybe an attorney would give you a free hour.

We signed notes & personal guarantees on all our notes.

Is the personal guarantee worth the paper it is printed on? 

It's no guarantee but it's a good place to start.

Income is guaranteed under disability and social security which, to my understanding is, a safe bet. 

In my listing, I make it clear that we 1. are clean freaks so we pay our cleaners more to guarantee our place is sparkingling clean and 2. pay our cleaners a living wage.

There are no guarantees in a short sale.

While there is never a guarantee (run if someone offers a guarantee), you want to tilt the odds in your favor with savvy choices.If you lack the knowledge and skills, start slow.