From the way you have described them, it actually sounds like you will have a great time. When MIL gets tired of walking and rides, if you are at MK, just drop her off at the Main Street shops and then meet her later. If you are at Epcot, she can spend all her time at Mousegears while you are in Future World, and then can have a fantastic time wandering WS and shopping all the different places while you are on that side of the park. We did a week with my Mom (high maintenance) and then to play fair for grandparent time with DD did a week with the in-laws 6 month later. I was a little worried about a week with the in-laws at WDW, because we don't travel the same at all. They are sitters, and we are doers. They are happy sitting in a hotel room for hours on end, while we usually are only there to sleep. We had a fantastic time with them. We told them going in that we had one TS meal a day booked for all of us, and other than that if they didn't feel like keeping up with us they did not have too. MIL wound up having to rent an
ECV because of an old injury, but they actually kept up with us most of the time and had a blast. When they got tired (or tired of us), which wasn't often (I can only think of twice) , they would just leave us and go back to the resort. It was a great trip. No stress or disagreements at all.
Now my Mom was not the easiest to travel with (she is constantly worried about something....DD is too cold, too hot, hasn't had a drink in an hour, is hungry, lunch is too late etc.), but we still had lots of fun. The way to get through it if it goes that way is deep breaths and lots of Grey Goose slushes at Epcot.
Both trips we had separate rooms that connected. I would recommend staying on WDW property, especially if you will not be in separate vehicles, because that makes it easier for them to grab transportation and just go back to the resort (or anyplace else) if they want, and meet back later.