I second going to your local mobility store. Tell them what your problems are and ask their advice. There are a lot of mobility aids not available even a few years ago. Whatever they recommend, you can probably rent for a day or two to get the feel of it. Grocery store carts are NOT the same as the usual
ECVs you rent. The rental ones are much more responsive and maneuverable. Grocery store rides are your basic tanks -- they are built to be indestructible. simple enough to be used by everyone, and designed to just take you up and down straight aisles.
Once you find something you like, compare prices renting at your local mobility store vs renting in Orlando. It may well be cheaper to rent it locally, plus you will have it for getting around the airport. (More practice!) I gate check my
scooter -- I ride it up to the plane door, then walk to my seat while they take it down and load it on the plane. (Mostly I gate check because I am paranoid about getting to Orlando and finding out my scooter did not.) This does rely on you being able to walk to your seat, though they do have special wheelchairs that fit between the aisles to take you to your seat. (But then you probably know that already.)
My experience at WDW is10 years out of date, but in most cases you will be able to go through the lines just fine. There may be problems at some of the Magic Kingdom rides where the rides are more dense and there was not room to retrofit the lines to accomodate scooters. Those will have a separate entrance. But be assured that WDW has a lot of experience dealing with scooters, and they are well aware that, due to the large size of the parks, some
ECV users will be first time or otherwise inexperienced users. They have planned, laid things out, and trained their ride staff to get you around easily
One last thing -- if you get an ECV, you may have a choice between 3 wheel or 4 wheel. I prefer a 3 wheel, as it is more maneuverable, and can make tighter turns. My friend prefers a 4 wheel because it has more stability and she has balance problems.