If she wants the
ECV (which is what you indicate in the original post), I'd go for that as long as she's safe driving one. The problem with transport (or any rental) wheelchairs is that the person using it loses most, if not all, of their independence. There's no way for the person in the wheelchair to move on their own (extra true with transport wheelchairs, although they are supposed to be easier for a separate pusher). While I'm sure your family would do their best, when being pushed you can end up spending a lot of time looking at walls or being told to look at something they can't see because the pusher hasn't turned the chair. It sounds to me like one thing your mother really does value is independence. It means she can keep up, but also go off by herself if she needs a break or doesn't want to do something everyone else is doing. If she wants to walk occasionally, she can also park it and walk through queues (I wouldn't do this for all queues, but some of the shorter ones might be okay), stores, or into restaurants. Doing a little walking may actually be beneficial because
There are spaces for
ECVs on all the buses. If she's worried about "parking", there are several options. One of the other members of your party could load it, you could stick it in neutral and push it into position, or on some buses the drivers will help. She'd need to move to a regular bus seat, anyway, so her not being on the ECV while it's loading isn't any extra step. She can also practice at your resort before you take the ECV to any parks. She is definitely would be able to use the smaller ECVs without problems and those can even be broken down to transport in a car if you have one. It wouldn't work to take it apart for the buses, but it's really not needed.
If you do take her to a store to practice, know that pretty much any ECV she could rent in Orlando handles better than the ones you borrow in stores. The ones in the stores at home will also probably be slower than one rented at WDW. I only mention the speed because if I'd first used an ECV at a grocery store, I'd probably have refused to ever use one for anything bigger than the store (most really crawl along).
Whatever you/she choose, I hope you all have a wonderful trip!