I guess for the wheelchair or
ECV question, I would have a few questions?
Have you talked to them about using a mobility device?
It's pretty obvious that they will need one, but if you have not mentioned it, it's better to bring it up now than spring it on them just before you go.
Have you asked them which they prefer?
Some people would be too nervous to drive an ECV and would be happier being pushed in a wheelchair, even though that gives less independence.
Other people are really interested in being independent and don't want to be pushed around.
Are both of them still driving a car?
People who are still driving a car usually feel more confident driving an ECV. If either of them never drove, they may feel more comfortable with a wheelchair.
Do any have hearing, vision or attention problems that would interfere with being able to safely drive an ECV? Driving an ECV does require a certain amount of concentration. Because there are no 'roads' inside the parks, people do wander across your path as you are driving. It's not uncommon for other guests to cross right in front of you without warning.
Have either of them driven an ECV before?
If they haven't, they can test one out in most large stores like Target and
Walmart. If they can drive one of those, which are large and have a big shopping basket stuck on the front, WDW will not be a problem.
You might also want to look into other options - like renting one ECV and one wheelchair if they both need one at the same time or using a rollator (sort of a walker with a fold down seat).
My FIL used an ECV at Epcot and at the Studio, but felt it was too congested and busy at MK and AK. Some people do feel that same way while others are comfortable at all the parks.