My family has been to WDW multiple times over New Years. It was fun, but you could not pay me enough to be in a park for New Years Eve.
I went to MK on NYE once with my older daughter; neither of us have any disabilities, but it was still difficult. Basically, we had no choice where we went. We just became part of the crowd and had to go where the crowd went. Trying to break off from the crowd was almost impossible.
We were shoulder to shoulder with people and had people almost touching our backs and could have touched the people in front of us without even stretching our arms out straight. I lost my daughter several times - we had cell phones and had decided places to meet up ahead of time. There was no place that was quiet or less busy. Long lines for bathrooms and food; when we did get a place to sit and eat, we had people standing eating their food nearly touching us.
I've also been to EPCOT several times with my husband and younger daughter, who is a disabled, full time wheelchair user. Also, a couple times just my husband and me.
Because there is more space, people are a bit more spaced out, but literally couldn't stretch out our arms without hitting someone. Not much fun for my daughter, anyone in a wheelchair,
ECV, stroller or even small kids walking because they will see a lot of butts and not much else. That was very difficult for my daughter because of anxiety when people are too close or she feels blocked.
The area around the entrance to World Showcase will be most busy because people want to be there for the fireworks (and getting out afterwards).
We stayed one time at Beach Club and the rest at Boardwalk. That allowed us to 'escape' the park and see the fireworks from outside. Boardwalk had a party with a DJ, which was more enjoyable for us. Except for one year, we left EPCOT early after picking up our party hats and never went back in.
I would not spend NYE at MK again unless I was at Contemporary where I could walk to easily. Also would only do Hollywood Studio or EPCOT if staying at one of the EPCOT area resorts so I could escape easily.
They are going to decide whatever they decide, but at least they can make a more informed decision