I haven't read to the end of the post so I apologize in advance if this is a repeat suggestion. To keep your quilt from "walking away" at the nursing home, make it as PERSONAL as possible...pictures of her & the family, names on blocks, first & last big & bold too. After all it's for her at the nursing home, & doesn't need to be entered @ the quilt show as a generic beauty. I don't mean that it should be ugly, just alternate your colorful blocks with a few personalized ones. You can get that inkjet printable fabric @ Joann's or any LQS & use a picture of her with one or all of her children...you never know what might jog a happy memory for her. Also, I agree, it could be lap sized for the wheelchair, I wouldn't do a whole bed quilt...it will end up going through the facility laundry if you do; there are a lot of accidents @ this stage.
My Grandmother was only in a home for 2 years & passed @ 96. At that point she didn't know us, but giving her sewn items with her "favorite things & colors" always brought her joy when we came to visit. That always warmed my heart, as it was "For HER" benefit, not ours. I realize your MIL may want this to be a beauty as it is coming from her, but even her name on a center front block will keep it from "walking so easily". Keep the end recipient foremost in mind. Best wishes & you are kind to do the work.