@Maistre Gracey,
ECV rentals are $50 per day at WDW. My concern about renting an
ECV at Epcot is getting your MIL from the resort to the International Gate since she cannot walk any more than a few steps. You may want to also check with the local
ECV rental companies to see if they might be able to rent you an ECV for just a day or two. I use Gold Mobility when I'm at WDW and they deliver to me at my resort. Their price is $230 for 1-9 days. But, since you are local you may be able to negotiate a lower price for 2 days (your arrival and departure days) if you pick up/drop off yourself. They're on 192 just east of Old Town.