I traveled to Disney regularly with a severe mobility disability and with elderly family members. just came back from a trip with my 95-year-old uncle at his 60-year-old autistic son.
The Boardwalk is not a nightmare to navigate. You're talking about family members who are going to be using scooters or rollators. They're not going to be using stairs. So they have a room near the elevators. That's all that's required. It will put them in the central location nearest the lobby or ground floor exit.
Yes they would be more comfortable if a family member or friend was with them as a companion to help them navigate, open doors, remember the schedule, and run interference. Disney World is not a third world country. There are a lot of amenities and it is extremely accessible across the board. Don't twist yourself in a knot over this.
My only concern is if you had a family member with dementia. As you know that requires a full-time companion to ensure they don't get confused and lost. But if their entire issues are mobility related and stamina related, you can easily deal with that.
I highly recommend anyone who is using a rollator rent a
scooter. Rollators are fine if you're going to go no more than a mile in a day. But Disney requires 8 mi or more of walking per day. There's just no way you can handle that with a rollator. I understand your concerns of scooters and bad drivers. Your best bet is to turn the speed down! It was on turtle, it is far less likely your bad driver is going to have a collision with somebody.
But if you cannot trust them at all with their own power wheels, then get them a manual wheelchair and enabled bodied driver. Don't make them walk it. They will be miserable and you will feel guilty.
The wedding coordinator at Disney will organize all the transportation to and from the wedding venue, the reception and fireworks in Epcot. They do this on a routine basis. In fact I saw several weddings last week being led to the fireworks viewing spot in Epcot between the UK and France. They do this with cast members in the front, middle and the end gathering everybody together and taking them through the park. I have absolutely no concerns they will get your family members to the necessary events.
Now my family uses a combination of Life360 on our cell phones to GPS track each other, and group chats to coordinate.