Welcome to the disABILITIES bb! I think your whole trip will be easier on everyone if your MIL uses an
ECV. She wil lbe independent & no one will have to push her (like in awheelchair).
I have a knee condition, & I rent an ECV while on vacation at WDW. I would definitely rent an ECV from an off site supplier. I have rented from Walker Mobilitiy several times. Someone else already listed Walker & other companies that rent
ECVs.
I started using an ECV at WDW by renting one at each park. I rented from Disney for a few trips & learned over time that renting from Disney presented a few "issues:"
*no guarantee the park will have an ECV regardless of time of day; I was at EPCOT Mother's Day 2002 a half hour after the park opened & no ECVs available at either entrance; another time I entered the Studios in the early afternoon on a weekeday in early December & no ECVs available;
*if you want to park hop there is no guarantee of an ECV at the second park;
*four wheels on a Disney ECV vs. three on off site one (at least the ones I have rented from Walker); three wheels gave me a better turning radius;
*the battery from Walker holds up a whole lot better than Disney one; I can tell you where in each of the four parks where my Disney ECV "died

;"
*if you rent an ECV you will have it every where you want it - the parks, Downtown Disney, other resorts, Sea World, Universal, etc.
As I mentioned I have rented from Walker Mobility. My experiences have been very good. The ECV has been delivered in a timely manner; the battery has lasted all day & night; the cost is similar to Disney & I have the ECV more than just in the parks; etc. Walker provides a battery charger that I use at night.
As for using an ECV - go for it! You have a need & the ECV will make your trip more enjoyable.
I used the busses to the parks. Initially I was hesitant, but the bus drivers were so friendly & very helpful. The bus driver generally closes the front door of the bus first & lets the guest with the ECV on through the back door with a ramp. There are two types of busses. Once you & the ECV are loaded & secure, then other guests are allowed to board the bus.
Have fun & enjoy your trip!!