If someone can not stand for long periods of time and has an ECV will she be able to wait in line on the ECV? She can get off and walk shortish distances, but she can't stand up for long periods of time- more than 2 or 3 minutes. She's willing to wait her turn, just needs to be sitting down while doing it.
I believe she'd be able to stay in the ECV in most queues. For the ones where she can't, she may be able to transfer to a wheelchair to go through the queue. Some rides have alternate entrances, Spaceship Earth is a great example. You still wait in line, generally just as long as the main queue, but they're able to slow the moving walkway so people with mobility issues have an easier time getting on and off of the attraction.
Each attraction is different, but I traveled with a friend who needed an ECV a couple years back. The best thing is to ask the attendant outside each queue and they will direct you to the appropriate entrance. The cast members are well-versed in how to handle this.
It is totally dependent on the queue's size. One's with a lot of switchbacks, probably not, but the wider themed queues much more likely.