One tip that might help her feel more in control of the
scooter is to make sure that the speed is turned all the way down to "turtle" (most rentals have a "turtle to rabbit" scale that indicates the relative speed of the scooter) when entering a queue. By going to full turtle mode, she does not have to worry about the throttle, only steering. It's great that y'all rent a 3-wheeler; those do have the best turning radius. The other thing she can always do is to practice queues at any of the Resort Hotels (often in the QS locations or sometimes the bus stops) and at Disney Springs. Disney Springs is a great place to practice, especially during the day, because folks there are not in "Park mode" where they want to go go go, and it can be intimidating to some people.
Also, please remind Mom that using an
ECV at Disney (or anywhere) is just like using a tool to get a job done. I bet at home, if she was hanging a picture on the wall, she would grab a hammer, right? If she needs to do math, she might pick up a calculator. And to see better, she might use glasses, or she might use hearing aides to help her auditory skills. Those are all *tools*. Just like an ECV helps someone walk further, or increase their stamina. Also, apparently in your Mom's case, she can add "Disney Royalty" to her list of things it helps with!

(I *love* that so much!)
It's possible for y'all to bring along a walker, or a Rollator (fancier walker with a built-in seat, and typically storage underneath) that can be carried on the seat back of the rental ECV (let your rental vendor know in advance that you need to be able to carry along an additional device, and what kind)
Then, Mom parks the ECV outside the ride or attraction, and uses the walker/Rollator through the line (Rollator gives her a place to sit if the line gets "stuck") and there are no worries about the ECV.
I understand her concerns - even after *years* of using an ECV at WDW, there are some queues that just feel a bit too tight. I have long said that the folks who lay out the queue lines should have to *personally* traverse the line - and it's tight corners! - themselves in both a wheelchair and an ECV to make sure that it works for everyone.
I hope y'all have the best trip you can - I love the mother/daughter trips that my daughter and I go on; they are some of my most precious memories!