Explain what you mean by highly coachable, because I'm not sure if I have the exact definition for it as you do. To me, a system that's highly coachable is something that anyone and everyone on the team can understand and take part in and the ones that can't handle it, aren't NHL quality players. The PK is not an area I would file that under... by far.
Look, at the end of the day, guys like Acciari, Kuraly, Wagner, Nordstrom, etc etc are all replaceable parts. The reason why they stick is because the ones that are better than them end up playing on higher scoring lines. The 4th line is and always will be the home of grinders, pk'ers, speedy guys with hands of stone, gritty sandpaper types, agitators, etc etc. A team's success depends on the all the parts functioning and it needs guys who are willing to do the dirty work. Typically a guy like Acciari is your callup from the minors or your spare 13/14th forward. Maybe they don't like the lack of preparation or the soft on the puck style of Bjork ( just saying it for the sake of the conversation ). Maybe they feel Acciari adds some sandpaper to a team lacking that sort of presence along the boards. Acciari's contributions are extremely limited but at the end of the day, if the team felt Bjork, Senyshyn, Cehlarik etc etc were better suited to play in the NHL, they would be here playing. I assume at some point, they will be.
I want my 4th line
1. playing with energy
2. hard on the puck and along the boards
3. defensively responsible
4. Muck and grind their way to scoring chances a couple of times a game
For the most part ( and especially of late ) our 4th liners are doing just that.