DO NOT CANCEL.
Now that is out of the way... If you think your Mom wants to drive an ECV, I suggest her "test driving" one at your local grocery store. These are about the same as those rented directly from Disney once inside the parks. If she is comfortable with that, I would suggest renting from an off-site company. There are several on the internet. They are cheaper than renting from Disney and you have it with you in the resort. Disney lists some companies as "featured" providers on their website. To be "featured", the company pays Disney a fee to be able to store the ECV at baggage storage at the resorts. "Featured" has nothing to do with their service or quality levels. There are numerous posts on the DisABILITIES forum that have reviews of both "featured" and non-featured ECV providers. I have used
Buena Vista twice in the past. I did get stranded in Hollywood Studios with a faulty battery during the first time. Next time, I requested a newer model (which worked great - but shouldn't have to request that). This time I'm renting one from Gold Mobility. Their rentals are less than
Buena Vista and they offer more accessories (included in the rental fee) than most providers. They are not a "featured" provider, so I have to meet them in the lobby to pick up the ECV. I scheduled the time when I made the reservation.
If your Mom (or you) are not comfortable with using an ECV, definitely rent a wheelchair from an off-site provider. Once again, renting one off-site is more beneficial financially. I second a previous posters suggestion of getting some type of gloves for the pusher(s) to wear. I made the mistake one year of not having any while pushing my Mom around Magic Kingdom during
MNSSHP. My palms were red raw the next 2 days. I ended getting some weight lifting gloves to use since they had a padded palm.
Most providers (ECV & WC) offer accessories that will hold canes, crutches, etc. I highly recommend paying the extra couple of dollars to get them.
Cast members are trained for dealing with guests that use ECV and wheelchairs. They will direct you at each ride (and attraction/show) where to go and if there is an alternate entrance or a specific location to "park". I've gone several times solo while using an ECV and have never had a bad experience with a cast member (including bus drivers). Now other guests are a different matter entirely. You will run into clueless ones not paying attention to where they are going, ones that are rude, etc., but you will also meet ones that will offer to assist and hold open doors and gates for you. From the sounds of your party size, you could easily create a protective barrier around Mom (whether she uses an ECV or a wheelchair) by having some of you in front as trailblazers, some on the sides as buffers, and a few in the back as bumpers.
I hope you still decide to go. Just remember, a bad day at Disney is ALWAYS better than a good day anywhere else.