Are they going to pay you back?

Honestly, the last thing I would do is try and return the car because I highly doubt Nissan has a 30 day grace period where you could return the car.

I would keep the Z and give them if anything 500 bucks
let them come up with the rest. Its THEIR PROBLEM. I know you want to do a good thing but by making yourself miserable and helping these debtors wont make things better. It may makes things worse because now they know they can depend on you whenever they dont feel like paying their bill.

Your wife on the other hand needs to have a better shoulder.
She has her own life to worry about and shouldnt be burdened with anyone elses financial problems. The next thing would be to sell your house to pay off her grandmothers bills.
The #1 person you should worry about making Happy is yourself.
Obviously your not pleased with this and maybe you should firmly tell your spouse that .