Not picking on you specifically with this, but I think folks radically underestimate how difficult it is to shoot free throws under game conditions, and how difficult it is to change technique on something that you've been doing for years quickly, and how much more difficult it is to change under game conditions. There's a reason why college basketball players have been hitting more or less the same percentage of their free throws for years and years. (The percentage has actually been going up very slowly over time, but I think that at least part of that improvement has to do with who is shooting them since big men with limited offensive skillsets are less involved in the game than they used to be.)
I think that Harris' problem is at least partially related to what it takes to hit free throws in games. The actual challenge in making free throws isn't in taking the shot itself. We know this because every idiot on this board, myself included, can wander out to their driveway and hit seven of ten with little effort. What makes it difficult is transitioning from the mental pace and physical effort of live ball play to a state of relative calm and rest as you shoot a free throw. There are whole sports dedicated to challenging athletes around this concept. Biathlon is probably the most famous, but I'm partial to chessboxing, personally.
Anyway, we know that Harris is a high voltage player. He's extremely engaged and puts up a ton of effort when he's out there, which is why we love him. But while that's great for defense, that doesn't serve him as well on the offensive end where you need to slow the game down mentally to put the ball in the basket. I'm guessing that at least part of his struggles at the line are simply related to an inability to downshift when he needs to, and that he's probably a decent free throw shooter under less stressful conditions. The good news is that I think he can grow out of that, but we'll see.
As for changing his technique, that's a tougher sell. I think there's a theory that when people are under stress they tend to revert back to what they've practiced most frequently. It's not ironclad, but it can explain why these kids might be working on some stuff in practice that doesn't show up in games. If Harris has a flaw in his technique, that means he's shot the ball that way thousands and thousands of times and it will take some doing to have the new reps he's getting up with new technique show up under the stress of game action.
I get that you want to make your free throws and that they have an impact on the game, but I don't get why some folks put so much emphasis on hitting them relative to other more important factors. It's almost like there's some sort of moral judgment on guys who can't hit them consistently, like you should hit 90% of your attempts or else you're not trying or you have some other problem. But it's just not that easy. If it were, we wouldn't see the NCAA average free throw percentage clock in at roughly 70% year after year.