Could it be that it was not a bad thing to take the approach they did?
Don't disagree with anything you wrote, but that precaution piece I would understand to mean being careful not to break the player's confidence.
While some point to Slaf's production as a sign he's lacking in confidence, I see a player that has bought in to a deliberate approach, one that has him focused on growing his overall game (at the explicit "cost" of a short term negative effect on his ability to be comfortable/fluid and focus on scoring). I also hear an athlete that sounds confident & even appreciative of the direction, yet still hungry for more/better output.
That tells me the precaution, while absolutely important to be mindful of, in this specific case has been handled effectively. That's why I don't see the case to be made that going the AHL route would've been better for his long term development... And quite potentially would've had a bigger cost in shifting his focus on scoring (it's one thing to be ok with low output in the best league in the world while working on your game... Ego drive would likely change playing a level below, where things are easier and you want to earn your ticket up) & away from the specific individual & tactical fundamentals they are helping him refine.
Granted, there's no way to 100% validate which approach "would've" been better... But if the benefit was to ensure his confidence remained intact as he acclimated to NA pro hockey, I think we can say that this has been accomplished with the direction they took.