http://trooperbay.com/anh-stormtooper-dave-m-helmet-decals/
There you go. ATA like many other helmets are derived from TE, so the decals for TE helmets fit a lot of other brands too.
AM has a rounder dome than FX, it's one of the few changes they made to the helmet when they went from making FX to AM (it's same people behind both kits)
TM should fit you pretty good. I'm 5'10 and 120 lbs and I had to make my TM smaller, not length wise but the circumference of some parts. You probably don't need to do that, it should be fine out of the box with standard trimming.
I have no trouble sitting in mine. What I do is try to position myself so my weight is heaviest at the gap between cod and butt, minimizing pressure on the plastic. Granted that I sit quite upright, but it's working just fine.
As long as it's done properly. A big trooper may need a bigger kit, and those usually come with their own special modifications to become centurion eligible.
I think the flapping comes from the fact that a heavy holster from thick and rigid leather is riveted on the belt with only two small straps. Not nearly enough to keep them from not flapping around. Also Han and Luke had straps around their thighs connected to the back of the holster to keep the holster from flapping.
I think swedes are bad at norwegian mostly due to ignoring it, and not trying to understand. But on the other hand, I might be worse than most swedes. But I've improved a lot since joining the garrison and trooping with norwegians.