I just stayed at the Dolphin in October/Nov (2019) and use a rented
ECV - had no problems. There are tons of elevators. I did have to hunt for the best door to go out of if I wanted to get over to Epcot or the Boardwalk area - some doors don't automatically open but definitely there are ones that do. I didn't have any idea about it before I went so I'm sure if I could figure it out you will also. I only mention it cause there are LOTS of doors and walkways around the outside and some will save you some time over taking the main entrance door.
I used the boats a lot - no issues at all. Also no issues with the bus. I think it stops at the Swan first so perhaps there is a higher chance of having all the wheelchair spaces occupied but I never had to wait for another bus. If it had been an issue I might have considered going over to the Swan to catch a bus. Wasn't an issue...and I cannot say for certain every bus goes to the Swan first. I was always the only one in an ECV waiting for a bus at the Dolphin and there was a marked waiting spot. I also used the paths to get to Epcot and DHS a few times without taking the boat - no problems.
I use an ECV for long distances (like touring the parks) so I usually didn't take it to go downstairs in the hotel to grab food or eat breakfast - however it can be a long walk from your room to the elevators so YMMV on that. It's a big resort. Plenty of space in the room for the ECV.
Loved this resort - if you're a Marriott club member the perks are great there and you might want to take a look at joining if you're not. 10 out of 10 I'd stay there again.