You can’t grind the cranks - the rotor fits on the cranks such that there is 20 mm clearance between the rotor and the inside face of the crank. The max standard for disk calipers is 19 mm, so at least 1 mm clearance from the caliper to the crank arm. This is sized closely because there is a standard here to follow on the caliper, and to minimize the outwards flare of the cranks.
I believe that Turtle had to grind his Triton frame slightly.
Yes the KH/Schlumpf hub ideally needs to be about 4 mm longer (2 mm longer each side), IMO. Again that will be reviewed in future. Next time around I want the crank position to be identical to the Moment hub.
Kris