That's a very interesting question. The problem is that IF this is a real kidnapping, they were clearly not smart enough to figure out the proof of life. So we can't count on them doing the smart thing now.
If she is dead, I think they will think they be totally panicked about life in prison, blaming each other for everything that's gone wrong, and will think they have 2 choices. One is to leave her where she is right now and run. The other is risk moving the body to the desert and running.
There is one option that I haven't seen anyone suggest - that Nancy despite her age and mobility issues could be a feisty and stubborn lady. She might be mad about the attempt to steal millions of her daughter's hard earned money, and refuse to tell them anything that would suffice as proof of life. My grandmother has 84 when she died. I can totally see my Grandmother telling kidnappers that no way was she going to help them get her granddaughter's money. And she would have meant it.