Hi Terry,
you say she has some experience, how is that been? Can she manouvre herself easily, make turns, combine the driving and looking around for other walking around etc? If not sure, go to a
Walmart, Target, whatever and try some more. Let her practice as much as she likes and thinks she needs.
An
ECV will definately give her more freedom of mobility. That being said, she should be comfortable and safe enough to drive it, otherwise it could be hell for both her, you guys and others. With all of the people around it's nerve racking driving around if she doesn't feel in controle or actually is in controle. Now that second is easier to judge (more of you can have a look), the first is really up to her. Some elderly don't feel safe enough on a ECV to use it in crowded places, no matter how good they are at driving them.
It isn't hard to drive one, but opinions differ. I know some elderly love them, couldn't life without them, some find them hard to operate. While driver experience can help, it has really to do with 'feeling' the ecv. If you 'feel' it, like you can a car, you feel safe predicting what it'll do, judging corners and run a lot less risk of making not so smart manouvres.
You will be allowed to drive the ECV on the bus for her. The drivers don't mind one bit, they don't want anyone to ruin the buss. Just like you.

So it's in everybodies favour to have somebody do that who is confident enough to do it.
One thing you might want to think about is lines. Is she capable to manouvre the ECV in the lines where she can take it along? If not, can she stand for the long time that might be needed? If both not, an ECV has real downsides for her. In that case combining ecv and wheelchair depending on what the plan is for that part of the day might be a thing, but that can get expensive if you need to rent.
I'm very pro ECV. As long as anyone feels safe on it and drives safe, it's a aid that gives more range of freedom than any manual chair can ever provide. It'll give her the freedom of getting around the resort alone if she wants to. It'll give her the freedom to keep up with you guys and go back to the resort if she wants to. She can get go shopping on her own if you guys are on a rollercoaster she doesn't like doing. My advice? Go and practice and take if from there. That'll tell what's right for her.