I would suggest calling Gold Mobility, they sell off their old equipment every 6 months after fully refurbishing them and replacing the batteries at a highly discounted rate. They will also sell new if you prefer and you can get their accessories.
If you already have your new scooter, still call them, they can help you with the accessories, the new cup holder they are using is great, they have a great cell phone holder and fan. Also, they can add the undercarriage lighting as well. I am not sure what the current price for the package is, it I think it is reasonable. They can also add the reflective stickers if you would like. They can also sell you a cane holder that works well. I do not recommend their sun shade, still trying to figure out a good solar shade in umbrella attachment.
As for the seat cover, If you got the captains chair, I would look at smaller universal car seat covers, that should do the trick. Again though, Gold Mobility may be able to help. If neither of those work, there is a group on Facebook called Ears for Each Other that has CMs that are still laid off or some ended up starting their own business and aren't going back, etc. And some may be able to make something custom, even Disney themed at a reasonable price.
As to the rain, unless it is really bad rain, just cover the tiller with a shower cap and you will be fine. This is what they recommend for their rentals. If it is really bad rain, try to find cover for a bit. Basically, what you don't want to get wet are the USB port and charge port or the tiller and controls. Usually your body does a pretty good job at protecting the ports and the shower cap protects the tiller well.
Also, there are safety mechanisms built in, so if things do get wet that shouldn't, the scooter won't operate until completely dried out, but likely won't be permanently harmed.
Hopefully that is helpful.
I should also state that DH has added a second cup holder to what Gold Mobility out on the one he bought from them fr HandleStash
https://handlestash.com/ they are meant for bikes, but work great on the scooter, especially over the cobblestones, etc. That occur at the theme parks, he tends to use both of those and the built in one (on the 10.2) gets used for regular sized drinks.