Sorry to hear this, as you said, not the most helpful timing

. The
ECV sounds like a very sensible idea indeed, and as Sue said, this generally works out a lot cheaper from external companies than direct from Disney. Also, with your own ECV you don't need to worry about rentals running out; they're generally a bit nippier and easier to drive; you have it to use at your hotel and for DTD; and, best of all, you can decorate it up!
Whichever company you decide to go with should be able to put a crutch-holder onto your
scooter for you, for only a small extra cost. This will make it a lot easier for you, as you won't need to find somewhere to balance them while you're driving, or get someone else to carry them.
You might want to have a bit of a practice driving at your hotel, before you hit the parks. You'll need to practice reversing into spaces and parallel parking, as you'll need both of these to do the buses. I'm sure you'll be fine, you just need to get used to the controls and your new 'body space' (I'm guessing with riding and show jumping, that your spacial awareness is pretty good already, so that should help).
Just be warned that you will encounter a fair few people who are 'off in their own little world', and will probably wander straight in front of you. You shouldn't have too many problems, as the crowds shouldn't be too awful, but just be aware that you have to look
everywhere while you're driving, because there are a few dingbats out there who won't!