I think she really knows deep down. Getting over the pride and years of thinking about things a certain way seem to be a big barrier. I'll certainly encourage more walking prior to the trip though and see if it dents anything
Please tell her this:
That there are literally *thousands* of people, every week, at Disney World who are using mobility devices
who never use them at home in their daily life.
Most folks don't walk between 3 & 10 miles per day at home, so they never bump up against the need to use a tool for mobility. And that's all a scooter is - a tool to get a job done properly. It's no different than wearing glasses to see, or hearing aids to hear better - it's just a tool to be able to move around with less pain and swelling at Disney World.
And no one will pay any attention to her, unless she decides to decorate that rental scooter with neon lights, handlebar streamers, flags, and hot pink seat covers. I promise you, her on that
ECV will be a total non-event. There are *so* many people using ECVs now that no one will pay any attention.
Let her know that she can use the "park and walk" method if she wants (where you park the ECV in a single location, walk around that area, and then move it on to a new location), and that she absolutely does
not have to take pictures with the scooter - she can park it out of sight before snapping that perfect vacation photo.
We have lots of guidance here on how to use the scooter (in general) if she wants/needs it for reassurance. She can "practice" driving the scooter-cart hybrids at local stores in the meantime; if she can master those giant beasties, a rental at WDW will feel like a Lamborghini (not exaggerating LOL)
Last but not least, let her know that you don't want her to need a vacation to recover from her vacation at Disney World! Tell her that you are NOT looking forward to watching her creep from bench to bench all day - and that there are now far fewer places to sit since the pandemic; a lot of those were removed for social distancing, and have not been replaced. Remind her that it's supposed to be fun and relaxing for *both* of you... and that you won't have much fun, or be very relaxed if you are worrying about her for the whole trip.
Let us know if we can help answer any questions she may have about using a mobility device at WDW.