I've got a "passable" golf swing. But I could spend every day for the next year tweaking it and I still wouldn't be on tour. Devin can certainly shoot better than 36% (over the last two games) from the line and I agree that the drop from around 60% to <40% is likely mental or fatigue related.
Any coach who thinks they can spend one afternoon with a guy and get him above 70% from the line is either bullshitting or not a very good coach.
It would be a waste of Devin's time to spend a ton of time on free throws. Putting an inordinate amount of focus on the issue likely isn't going to help with the mental pressure and his form is not going to get better enough to make a significant difference. Even if Devin somehow jumped from 55% to 80% from the line we are only looking at around 2 points a game.
I'd rather him spend that time battling with Dre, Moore, and JC in practice or learning how to pass out of a double team. He's more likely to find 2 extra points for his team in those areas than he is at the free throw line.
There's a lot more between having a good swing and getting onto the PGA tour. But I bet that with an improved swing/grip, you add some more control and distance to your shots, which would decrease your strokes per hole. Much like being a decent free throw shooter doesn't make one a NBA player. For being full of shit or not a good coach, there are somehow numerous free throw coaches, who have good results and get paid nice money by high school, college, and NBA teams. Rajon Rondo is one example of this. No clue how he is doing now but he used to suck, then spent time with a "free throw guru" and started shooting much better.
While free throw shooting does require some time for practice, it more requires having proper form and a proper approach, which can be quick depending on how long it takes to absorb the new approach. Otherwise, you'll be spending hours upon hours in the gym simply reinforcing your bad habits which won't help anything.
I've been fortunate to speak with a few successful former coaches and they would each agree that improving free throw shooting is not some impossible task that some make it out to be.
Some issues with Devin's free throws:
1. Low arc - less arc means a lower chance of making it. The hoop is big enough to fit two basketballs at the same time, lowering the arc greatly decreases the hoop area
2. Wanders after the shot - rather than stay in place, he moves, which often results in an improper follow-through and bad spin on the ball
3. Low backspin on the shot - backspin helps soften a shot so when the ball hits the hoop, it doesn't go bouncing off but gives it a chance to fall in
Devin is not that big to have DeAndre Jordan hands, where his hands are simply too big to be that good at free throw shooting.