It kind of seems as though you are ruling out suggestions without looking into it, but it's hard to tell from your posts.
If they are willing and able to put some $$ and effort into it, I do think they can make this work. How much $$ may be conversely dependent on the amount of effort -- meaning less effort = more money.
- Contact an offsite vendor to discuss options. Gold Mobility gets mentioned here a lot, but Gold only rents for a full week so that may be much more than these ladies need. Maybe try Walker Mobility, Randy's Mobility or Orlando Medical Rentals who do shorter-term rentals. Any of these offer at least a couple options.
- If they cannot break-down a portable ECV, they can use an accessible Mears taxi. I'm not sure if Mears has one that accommodates 2 ECV or not, so it may mean 2 taxis. They can get that information by calling Mears, and probably an estimate of the cost. Regarding cost - a taxi may only be a bit more than they would pay for parking, and the taxi will drop them closer to the entrance (or for MK closer to the ferry). Walking the parking lot can be brutal, even if they can use HA parking.
- If renting an ECV is too much cost or effort and they still want to go, I strongly recommend getting there as early as they can. In-park ECV rentals can run-out by mid-morning. And they should have a "plan B" in case they run into that very situation. Maybe pushing an (empty) wheelchair like a walker and having it to sit when needing a rest. Hard to suggest without knowing their specific capabilities/limitations.
Your first post also mentioned
Genie+ and LL$. Depending on when they plan to visit, those may be a good idea but not required. Especially if they won't be interested in the higher-thrill rides (coasters, etc.).
If they are truly 3 little old ladies who can't lift anything heavier than a bag of flour and can't walk more than to the end of the block -- unfortunately WDW may be much more of a challenge than they realize. I'd hate for them to have a bad experience.