You're suggesting he increase his FT% from 52% to 75%. That seems like a stretch. Sanogo is one of our most improved shooters EVER, and went from 58%, to 69% to 77%. I think a reasonable goal for Clingan would be mid-60s at this stage with a long-term project of getting to the mid-70s when he's in the league.
You're generally right... his release is sort of consistent--he's missing because he has no arc.
He holds the ball too far from his face and doesn't start with the ball at his hip. His life is too far from his body and he tends to hold the ball on the center line and doesn't get the center of his hand under the ball. His lower body looks pretty consistent though, IIRC.
No, you can't fix all of that. No one's FT is perfect. As long as he gets enough arc and shoots the same shot every time, that's all that really matters.
If I'm coaching him up on FTs, I'm going back to basics. That growth spurt through everything out of whack. Hand position on the ball, start the shot on the hip, keep the ball close to your body. working his way back to the FT line from little six-foot jumpers. Forget # of makes or takes--just take shots with perfect form.
I suspect he'll be working with some high-level NBA guys this summer like Hawkins did last year. Should get a ton of great feedback.