Dubas wasn't getting groomed by Lou.
Lou and Shanahan right after Lou was hired.
10 questions after Lamoriello hiring - TSN.ca
Lou pretty quickly realized they didn't see eye to eye and forced him into a small corner of the organization (which was Marlies and the analytics team). He also promoted Hunter to be on par with Dubas to make that statement.
Absolutely no substantiated proof of them not seeing eye-ot-eye. Additionally, a lot of AGM's take on the responsibility of GM for the AHL squad, including many of the AGM's that eventually became GM's. Were they all forced into small corners of their organizations.
As for Hunter, none of his duties changed. He simply received a change in title.
Maple Leafs name Mark Hunter Assistant General Manager
When Hunter left, we hired a new Director of Player Personnel to replace him (Wes Clark).
That's why Dubas tried to force his way into a lateral move to Colorado which Shanahan blocked it.
Shanahan did not block it, and there was nothing said about it being a lateral move. There was no formal indication of what it was, only speculation that it could have been that he replaces Sakic as GM while Sakic continues handling his other duties within the organization.
Maple Leafs grant Avalanche permission to speak with Kyle Dubas - Sportsnet.ca
There were also rumours from Friedman regarding Arizona having interest in Dubas as GM, but that he had no interest in becoming an NHL GM at that point. The Coyotes owner later disputed those claims however.
Then when push came to shove, Shanahan chose to move on from Lou to promote Dubas.
When push came to shove, he made the decision he felt was the best for the future direction of the organization. Not that you have said it (but many others with your position have), the Leafs did not fire Lou. They chose to not extend him and were following the plan that was expected from the day Lou was hired. Lou may not have liked the outcome, but he certainly knew from day 1 that it was the most likely scenario to occur.
As for what Dubas had to navigate, he had to navigate figuring out how to pay 3 of his young core players and did so while also managing to add another franchise center for nothing but money by making the team one of the most cap efficient in the league. That was a retool, and one much more difficult than many other GM's have had to deal with. He managed to make teams that are a lot more competitive than Lou's team (which was, as I said before, boosted by shootouts and near perfect health) despite facing immaturity, inconsistencies and injuries. He had 4 more ROW last year than that 105 point team, and under Keefe (with the same roster) is on pace to match that 105 pace with more ROW's. He still has a lot more work to do, but the work he's done is a whole lot better than the negative recognition he is currently receiving suggests. It is simply blind bias that people are failing to even admit they have.
