Seems like you've gotten alot of good informtion here for most of your questions.
I have one other suggestion, though I know it's a bit controversial. I had similar enormous concerns about my DS (ASD) on his first flight (in 2004 as the TSA was really ramping up). After much nail biting, consulting, conversation, etc., we opted for medicinal intervention. With the confined space issue and all the stimulation, it seemed 50/50 to me whether we'd be allowed to stay on the plane through take off, let alone the whole flight. Our doctor made a suggestion, we tried it two weeks before the trip to make sure it had the desired effect (awake, able to walk, but mellow and fell asleep easily) and it was good from there. Make sure you try it before the actual event because sometimes medicines can have an opposite effect on our little ones.
As an aside, after that he's been fine on plane rides. He remembers being very calm the first time, so it's what he expects now.
You know you're kiddo best, just wanted to give another option.