Please consider that people in wheelchairs do this same thing, every day, without event. The parks (and the resorts and transportation) are set to accommodate people with disabilities. In fact, there are laws that dictate that most everything must be accessible- and Disney often goes beyond what is legally required.
Transportation from the Polynesian to Magic Kingdom can be via monorail (where they have CMs who put down a ramp onto the monorail train) or via ferry (where they have flat ramps onto and off of the ferry). You can get to Epcot using a different, but identical, monorail track. Hollywood Studios and Animal Kingdom would require you to ride a bus- and I think several of them have lifts.
There is a guide for the rides in the different parks and how accessible they can be- somewhere on this forum. For most rides, you will enter through the FastPass lane (with your party) and get onto the ride where most people exit. On the occasions when you cannot stay in your ECV, it will only have to be 10 feet away or so- but, in those cases, you can transfer to a manual chair. You will have an opportunity, when you arrive at a ride, to ask the CM about how the ride is structured, so you can make a determination about what you can do. The CMs are almost always incredibly considerate and helpful in this regard. For the most part, there are only a handful of rides, in any of the parks, that are difficult to access- but, it all depends on your particular needs.
The dining areas can be tight because of crowds, but if your partner can "scout out" seating that will work for you, you can stay on your ECV to eat. Other than that, navigating around common areas are simple.
Also- dare I say it?- using an ECV is a whale of a lot more FUN than trying to hobble around on crutches or rely on someone for mobility. You should definitely take the time to whiz around your loved ones while you have a chance!
I hope you have a great trip...