I feel nothing but compassion for Fultz, who seems like a really nice young man, and also a very gifted basketball player.
I went through struggles trying to adjust my jump shot in college ball, with negative results, and eventually returned my form to how I shot in HS, when my shooting percentage improved.
Personally, I learned through the years that the most important thing about your mechanics is to not think about it. Mentally you should just be connecting to the confidence you feel when you are about to take a shot, knowing it's going to go in before you take the shot. When you think about your mechanics, you tend to tighten up, which causes you to miss. This is equally true when throwing a pass in football, or pitching in baseball.
Your brain is either your best friend or your worst enemy. Right now, it is Markelle's enemy.
Watch guys like Reggie Miller shoot. Horrible mechanics.
Watch Steph Curry shoot. He often walks away before the shot goes in, because his confidence told him it already was going in.
I'm not saying that good mechanics are not preferable to poor mechanics. But unless you are trained at an early age with good mechanics, you are probably better off just leaving them alone, particularly if you have been able to achieve great results with the sub-par mechanics.
Fultz should be reminded that he was the number one player selected in the draft with those mechanics, so he should feel confident that he can excel at this level with them. His greatness was in his ability to both shoot from the outside and penetrate and finish effortlessly. That guy is still there. He needs to go find him.
Hope he does.