Have you asked grandma what she might like? Shown her options of what the resorts have?
My parents are 71 and 67. My dad is active and looks to explore,, but also likes to just sit and relax and enjoy scenery. My mom uses a
scooter, but tires easily due to health issues. She likes to check out shops and loves finding a good place to sit and read. The only
DVC they have stayed was BLT and they really enjoyed it. The walk to CR for food/shops was great and if the trip hadn't been short, they probably would have taken the monorail to Poly and GF. They did say that if the weather had been cooler, such as late fall to early spring, the would have liked to stay at SSR so they could just head over to DS occasionally. I know they would love to stay at AKL with savannah view, but I could also see them not leaving as often and could get a bit bored.
View of room would make an impact as they could then relax on their balcony/patio when they just wanted to relax/read. We needed a roll in HA room at BLT for my sister so they were spoiled with the hybrid view of MK and Bay Lake. At SSR, I know they said they would want a room near the walk to DS and facing DS. So, I do not think they would enjoy BC as much since there is not much room view options there. I think they would be okay with a BW view or of the waterways to DHS. I think Riviera would work as well since they would enjoy the views of the lake and CBR and be interested in check it out or taking the skyliner over to the Epcot resorts.
Something else to think about, would she be making her way to catch up with you by herself. If so, MK or EP resorts might be easier since she would have alternate transportation to some parks instead of just buses and only one bus stop option when using the bus. I love my parents, but they have gotten many places by sheer luck and backtracking. I could see them easily getting off at the wrong stop at AKL, SSR, or OKW.