Right, given that customs situation it's understandable. And if you can't find any domestic bike parts company that imports these things, you're limited indeed. Too bad.

Brake upgrade: I see no need. The original works very well. Brake power depends on both disk size and wheel size. With the tiny 355-size Birdy wheels and relatively big 160 mm rotors, you get good braking. It corresponds to something like a 260 mm disk on a 29er wheel, but they are often happy with "only" 200 mm.

RT86 with opposite rotation: Yes, potential problem. Should not be run backwards, and may not fit when flipped. Try before you buy