Eeee-va, thanks for your advice too! So GCH has spare keys?! That is a good tip so I won't panic but hopefully I won't lose the key. How did your mom lose hers? I can imagine forgetting it in the scooter! Also, I did read that about getting cold but had forgotten so thank you and I'll be sure to bundle up at night. I wonder if it would be "safe" to leave a throw with the ecv if I walk on a ride? I guess it's like strollers, I'll bring one that I don't mind losing in case it gets stolen.
I have no idea how Mom lost the key; she may have taken it off while we ate, but we don't really know. We did like you're thinking of doing--park the ECV, then walk around the land--so there were stretches of time when she wasn't directly using the ECV, which would make it possible to misplace the key.
But if you lose the key I wouldn't bother looking for it for more than a few minutes before calling for help. We wasted probably 40 minutes looking for it--most CMs weren't really helpful at all, and a few tried their best (calling Lost and Found and City Hall, etc.) but still couldn't do much. I think Mom and I decided to call Deckert's ourselves; the CMs didn't suggest it--but I don't think it's a situation the CMs deal with much. (While Deckert's surely deals with it at least once a week!)
(Once we received the instructions, I had to walk to the GCH and back, but I spent less time doing that than we spent looking for the key!)
And yes, bring a throw/something warm you don't mind losing. Even a sweatshirt (to wear or keep in her lap) would have helped. Mom actually put my heavy bag on her lap to try to stay warm!
We did leave some minor stuff in the ECV (nothing intentional except some leftover food and my jacket sometimes) and never lost anything, including when Mom accidentally left a present she'd bought for me, still in the Disney bag. IMO theft is always a possibility, but if you don't leave anything that looks remotely valuable, I don't think it's a PROBABILITY. It's not easy to push an ECV like it is a stroller.
As for learning to drive it, I would suggest you go to a grocery store/Target/etc. and try out the ECV there first. Mom did that (maybe 30 minutes shopping at Target) and then test drove the rental in the hotel lobby (we arrived maybe 6 or 7 PM, so it wasn't busy down there) and she handled the ECV pretty well after that.
If you're in a large metro area, you can do what my mom did and go to an area of town you don't normally go to to try out an ECV, if you're worried about seeing someone you know.