I would agree with your idea of get Fastpass Plus for the Litle Mermaid and go to Dumbo at rope drop. Either attraction is not likely to have a very long wait in May at rope drop.
Under the Sea has a wheelchair accessible 'chariot' that seats one wheelchair and one companion. Guests with wheelchairs or mobility devices wait in the regular line (or Fastpass line if they have Fastpasses) until close to the boarding area. There are some interactive games in the line, which are mostly wheelchair accessible.
As the line reaches the boarding area, they and their party are routed beyond the regular boarding area to wait for the accessible vehicle. How long you would wait there depends on how many guests ahead of you are waiting for the accessible vehicle. The ride itself is about 5 minutes and the attraction has a large capacity because it is continually moving.
Dumbo has an air conditioned waiting building with play areas for children (no much for adults to do in the play area, wheelchair or not). There is seating and wifi for the adults. The Fastpass line goes thru part of the building. If you don't have Fastpasses, you are given a 'ticket' (restaurant style pager) that goes off when it is the time for you to board.
Dumbo has an ADA transfer vehicle, but it is not one the wheelchair can go into. The ADA Dumbo looks pretty much like the 'regular' ones, but the side life up and out of the way for boarding. Depending on the height of the wheelchair, the seat may be a lot lower or close in height.
The ride itself is about 1 minute, plus about 30 seconds for getting on and for getting off.
This is a map of the area. The closest way to get from one to the first there is to keep walking past the Little Mermiad - lots of people forget that entrance and go all the way back to the teacups.
