We'll be going to Disney World in 10 days
My dad needs double knee replacement surgery and will be riding an ECV. He can't stand for long periods of time.
MCoryB pointed you toward the disABILITIES FAQs thread, which should answer all your questions. Post one of that thread is an index, which lists which post has which information.
With an ECV, he will not need a GAC at all.
Guests dont need a Guest Assistance Card to bring an ECV or wheelchair into attractions. With an ECV or wheelchair, he will not need a GAC because CMs will direct him to go to the accessible area.
ALL lines are wheelchair accessible all the way to the boarding area, so he will not need to stand.
Exceptions are Swiss Family Robinson Treehouse and Tomorrowland Transit Authority, which require guests to be able to walk. The Treehouse is a walk thru experience with many, many stairs. Tomorrowland Transit Authority has a steep moving walkway ramp (kind of like an escalator without steps) to get up to the 2nd floor boarding area. If he is able to stand on a steeply sloped moving walkway, he will still be able to ride.
There are some attractions which are wheelchair accessible, but not ECV accessible. For those, he should talk to the CM at the entrance and let the CM know he will need to borrow a wheelchair to get to the boarding area.
I don't really understand - are there some rides that will let you in an alternate entrance or a different line or something? All rides? All of the time? I am just trying to have a grasp on this before we leave. Should we still get fast passes for things, or will it be necessary?
We are a party of 8. 5 adults, 3 kids. Seems like I've heard that only a party of 6 can stick with the one with a GAC. Are they ever lenient on this?
Most attractions have lines that are accessible thru the regular line (called Mainstream Lines). There are a few attractions where guests with mobility devices will go thru the regular line, but then board somewhere else, because the regular boarding area is not accessible.
There are a few lines that are not accessible, so they have a different entrance.
The attractions have a greeter at the front, who will tell you where to go. You can also get a special park map for guests with disabilities, which lists the way to go for each attraction (Mobility Entrances).
Post 11 of the disABILITIES FAQs thread has information on Mobility Entrances
and the special park maps.
For those with different entrances or boarding areas, you may need to split into smaller groups because of the number of guest who can fit on a ride car or the space for guests with special needs is small.
For example, if he wants to be in a wheelchair for Its a Small World so he can avoid stepping down into the boat, the special wheelchair boat will only hold 7 guests (6 seats and one wheelchair). For Toy Story Mania, the ride car that stops at the accessible boarding area holds a total of 6 people (or by removing a seat, 5 plus one wheelchair). So, your party of 8 will need to split up.
You should get Fastpasses if you want to avoid waits (GACs are not meant to shorten or eliminate waits and the card does say to use Fastpasses if you want to shorten waits).
You will also want to look at posts 18-21 of the disABILITIES FAQs thread, which gives more information about steps up/down and actually boarding attractions.
Post 28 has information about attractions with moving walkways or stairs. Guests with wheelchairs and
ECVs will be routed to bypass the stairs, but will want to be aware of the attractions with moving walkways.