The only problem with trying to select a "quiet" resort is that any resort is only as quiet as your immediate neighbors. You could have noisy neighbors at any resort. I once had a group of six adults carrying on a loud conversation in the hallway outside my room at 4am - outdoor voices and all! Or the day it was raining and a group of kids decided to play dodge ball in the hallway outside my room - continually banging into my door. I generally travel with a white noise machine so I can drown out any noise when I'm sleeping. I would agree that OKW and SSR are generally quiet, as well as VWL. I did not have any noise issues at BLT, but I was the next to the last room at the end of the hall, so I didn't get a lot of traffic outside my door. Whatever resort you choose, request a "quiet" area - this will generally be away from elevators and other common areas where people tend to congregate.
Now, as far as which resorts would be best for your party size and health concerns . . .First, which park(s) will you spend the most time in? You should pick a location closest to the park(s) where you will spend the most time. If I know I'm spending a lot of time at Epcot, I stay at one of the Epcot resorts. If I'm spending more time at MK than the other parks, I stay near MK. If you are planning to spend your time fairly equally between the parks, then the resort location isn't quite as important. Does your MIL have issues with getting on and off buses several times a day? If so, consider a resort where you have monorail or boat transportation for at least two of the parks.