I don't t see why they can't have a single player exemption of the CHL-AHL-NHL rule. Allow one player per AHL team under the acception that they've played one post draft year. Something like that. So they don't bleed the CHL of talent but still have a rare spot for rare players. It'll also allow the AHL to have better progression of their prospects when you get the odd Droiun or Marner.
One problem would be that you could have one CHL team that loses four or five players to different AHL teams. It would be great if there was some way to let 19 year olds who've outgrown junior into the AHL, but there really seems to be no practical way to do it.
In Marner's case I don't think we've got anything to worry about. Worst case, maybe he's not quite ready for full-time NHL duty next year. So we just bring him along slowly. Give him sheltered minutes, maybe alongside someone like Komarov who is good at getting the other team's attention and keeping it off his linemates. Marner gets to make a slower adjustment to the NHL, and he gets to hit the gym with Leafs trainers full time. It's not a perfect solution, but it gives the Leafs full control over his development.
And this is all assuming that Marner can't step in and make an impact next year, and I really believe he will.