AvatarNicholas Walker

Why do your toes come of the ground? I think you weight it too much in the back, try squatting while stretching your arms and holding the kettlebell there, this counter balance is very effective.

The buttwink, well I have seen worse ( I was a lot worse FYI ). What helped me was clearing up the glutes, specifically the TFL region and hams.
Working the TFL allows you to create more torque, more ext. rotation and overall stability, definitely worth a shot 
Your background in skiing could have made you forget about your glutes, as you stated. 
My advice: Attack your glutes/TFL, try different techniques, mobilize them in every way. See the difference.
Work the hams, they might play a role in rounding the back, however you do go pretty low, which is very awesome!
Anyway keep up the great work, Share your progress!