Questions

Just a comment on 3..

Assuming you're going with an elytra launcher design that uses boats (well, their collision to be more precise) I'm actually not sure if that'll work as there is still some minor lag backs during certain situations and I'm not sure if those have to do with speed. For example, if I go ~10 blocks higher than the build limit (which is Y 256) it'll kick me back to Y 263 and make me lose momentum. Also if I shoot myself with a Punch II bow 3 or more times in quick succession while in flight I seem to gain too much momentum and lag back and lose all of that momentum.

Entity-wise you should be able to build a good enough elytra launcher, although perhaps not a 'super insane' one that shoots you up above Y 400. Boats are entities, but so are item frames and mobs. If you can have 50 item frames in a chunk surely you can have 50 boats in a chunk. As long as you don't go completely overboard it probably won't be a lag machine. But alternatively you can just have a piston translocation elevator going up to the sky limit and jumping off there. Or better yet, jump off anywhere and shoot yourself with a Punch II bow. No need to launch yourself high straight away as you can gain altitude doing this (correctly).

I wouldn't worry about getting banned for going too fast or whatever. i've flown from spawn to my base (10k blocks from spawn) often hitting the maximum speed I could by shooting myself with a Punch II bow without touching the ground once using the elytra, but regardless of that, if you're not breaking any rules I wouldn't even worry about being banned.