At age 20+? How is this really going to work?
An NHL job and his career should be enough motivation. This whole idea of coaches being able to motivate players by yelling and them or benching them isn't a good one, IMO. In the vast majority of cases, the player will pout or play better for a couple of games, before returning to his lazy play.
So physically, sure, the players are not completely developed. Mentally? It is mostly all there. They will have the training habits they have; change will come organically, not from a threat.
In regards to in-game adjustments....sure, you want your players to be versatile. You want them to be able to succeed against different players. But there won't be the urgency of making an adjustment in order to win. Like, for example, if Sommer has thrown out a bad combination out there, and it has cost his team 2 goals, while an NHL coach would immediately yank them, Sommer would keep them out there to see if they can figure things out, or in another way make them find ways of working with each other. The fact that doing so costs his team the win is less relevant.
Just a hypothetical!
Mentally, no male is truly mature at the age bracket we're talking about. Discipline is hardly polished in the early 20's no matter what kind of motivation you have for a glorious type of career.
As for your less relevant argument, I agree that it is less relevant in the sense that it is not very heavily dependent on results for the AHL coach but it is very much relevant teaching the right way to play and having the right culture to develop in which means having a coach that does treat the game and his players as if it is the same thing as an NHL game because that's what they're supposed to be grooming them for.