Most things are genuinely accessible.
The whole park is fairly wheelchair-friendly, we use a Mountain Buggy but see no reason you shouldn't get a Major through the same gaps!
Your issue with the major is that it doesn't obviously "look" like a wheelchair and on some rides you have to leave the buggy some 20 or 30 feet away and carry your LO to the start of the ride as you're not allowed to clutter the boarding area with buggies. I'm not sure if you'd have to do this if it were a wheelchair, and it may be down to the level of training/common sense of the individual staff member, of course. Similarly, you may have to ask them to slow the ride down to get on and off if it's constantly moving, or ask for help - it won't always occur to them.
The only ride I can think of that isn't accessible off the top of my head isn't appropriate anyway - you have to stand up in it - and that sort of guidance isn't posted outside the rides. It is in the small print of the "guests with disabilities" guide but, again, although you are obviously disabled, they don't offer you this unless you very specifically ask and then they have to have a good root round under the desk in guest services to find it.
If getting in and out of things is a pain, you can get her pre-measured; go down to First Aid, and they will measure her height once and for all and write it on a bit of card for you so you just wave that at the start of the ride rather than having to prop her up against a measuring stick each time.
Note what they say on the web site about the exit pass - it is most definitely not open to most wheelchair users (the queues are intentionally made wide enough that folk in wheelchairs can queue), but rather to peeps with diagnoses like ASD who can't do queues. So you need to bring written proof of that with you to get the exit stamp.
Other thoughts about the stamp - you can get 3 other people to queue jump with the person who qualifies, but they have to be stamped at the same time as the main exit pass holder. You can stamp as many people as you like, but they have to take turns to jump the queue. So if you are meeting people, meet up before you go in and all get stamped together.
Oh, and they do a 1-carer-goes-free thing so you shouldn't have to pay for your own entry.