I have put many, many, miles on my XB's and I'm curious, why another new rotor at only 20Km? Thats just 11K Americans, and they should last MUCH longer than that. BTW they are notorious for NOT being warped, but depositing brake pad material unevenly and feeling exactly like it is.... A good cleaning and re-bed typically solves that pulsing, and much cheaper too
The intake grate by the gas cap shouldn't have any direct path to the airfilter unless the actual upper airbox piece is missing? Yes the later ones have holes in them but you should darn near be able to put a hose in there and not kill the motor at highway speeds. I don't recommend it though! Haha. A re-useable filter (like KN) will work better when wet than a paper OE one as well.
The full line of the gas tank is the LOWER lip where the cap seals, the well has a drain and will just dump out, try to not top off above that or just fill up on the kick stand (not upright). Should solve your over-filling issue.
Happy to hear you're riding it and getting some miles