You'll want to check out the sticky on the top of the disABILITIES part of the board. Sue has posted a great list of which rides have which types of entry. On general, most lines are mainstream accessible meaning that somebody on wheels can use the same entrance and line for rides as those walking. For some rides the person will have to transfer to a wheelchair (which will be available at such rides), like for instance POTC.
Rides that have an alternate way of entry for those on wheels will pose a situation in which you'll have to count on splitting up. They will put a max. on the amount of people allowed to accompany the person on wheels. Max. is 6 included said person. Same goes for boarding the busses through the back entry.
As far as shows and parades goes; same thing again. If things get really busy, they might even reduce the max. number down even lower to be able to allow as many folks on wheels as possible at the specific handicapped spaces. Having said that, I tend to not use those areas for the parades. I find them not in the greatest places and if arriving on time, there are a lot of other spots available with a lot nicer views.
Some shows in theaters like for instance Muppets 3D, Honey I shrunk the audience, Fantasmic! etc. will only have a very limited amount of seats available besides the wheelchair/
ECV spots. Haven't counted at all of the shows etc. but have seen those with 1 to 2 seats per wheelchair/ECV spot. At some of those you will find plenty of seats right in front of these spots, but arriving early (for instance at Fantasmic!) might be the strategy you'll want to follow.
If desired, the person may transfer out of the ECV onto a regular seat at shows etc. If need be, the CM's will drive away the ECV and get it back to them after the show. Good example of this is for instance Festival of the Lion King, where there are a limited amount of wheelchair/ECV spots. Being able to transfer to a regular seat would open up a lot more space, reducing the chance of not being able to enter because of full wheelchair spots.