It is hard to say which I would choose, if you do EPCOT, you will not see it all, and should try and plan what you want to see before you go. MGM is more do-able in one day.
I like the idea of doing MK for a whole day, and parkhopping between two of the others!
Would Grandma be opposed to a wheelchair? My 81 year old mother spent ALL day at EPCOT with us Thanksgiving weekend - we did SOARIN, Figment, Spaceship Earth, HISTA, Universe of Energy and walked all the way around World Showcase. We rented a wheelchair for her just in case... she used it as a walker for the stability rather than sitting in it to be pushed. My DD8 was the rider - it worked out well because it was a long day, and at one point she actually fell asleep in the chair!