Being "an older person" myself, LOL, I can just tell you what we do. We both have some medical issues, so that might change things for you. First, we stay at POFQ for the same reasons, ie: easy to get around. However, with only one bus stop it might take longer to get to the parks if there are more than 2 ECV's waiting.
We are morning people, so we are up and gone before rope drop. We eat our biggest meal at noon, then go back to the resort to rest and recharge our batteries,
. The it's off to DS or another park for the evening. At our age, we are not morning to night park people, so I really doubt your gramma will be able to do that, even if she says she only wants to ride rides and be at the park. An afternoon rest really helps with fatigue and the aches and pains of getting older. No one escapes those pains. We also find we like to go at a slower pace and check everything out. We like the rides, but also love the shows, restaurants, street acts.
I just think that, even though she said what she said about the parks, she will probably need a break each day.
My daughter came with us on our last trip, and she is not a person to rest. So,, she headed to the pool with a drink and a book while we rested each day. Maybe that will help you out, or you could go to the park alone for an hour or two.
We have both used ECV's before at various times, and it is NOT easy to navigate those crowds with kids and grownups stepping in front of you and stopping. Also you are at bottom level and can't really see anything but that, and watch for other people. So take that into consideration as another factor that could make her more tired than usual.
We always have a great time at our slower pace with rest periods in the parks and at the resort each day. Even with that, our feet and bodies are aching from a day at the parks. Even "younger" people feel the aches and pains that they never would away from all the miles we walk at Disney. Hope this helps out, even if our actual experiences are not what you hoped to hear. You will have fun anyway and adjust; afterall, it's Disney