If Leafs want to develop Marner into a center not a winger like Kane then he will need plenty of time in the OHL and even the AHL practicing face-offs and the position outside of getting bigger and stronger.
If Marner is the next Kane then perhaps he can arrive earlier on the scene if the plan is to develop him as a winger.
He's coming off 1 season of dominating the OHL and about to have a 2nd.
The 'plenty of development' he needs is as a hockey player, not simply some positional obsession based on Leaf fans needing to know they have a #1 C.
Players determine their position with their play. Marner will be too advanced for the OHL to play there at 19 no matter what position he's playing.
Since he can't play AHL at 19 the only option is NHL action outside the OHL. Since you want to shelter him, you set realistic expectations for training/conditioning and hope to get him into 60 or so games with spot duty and if he earns more or exceeds those expectations then that's even better.
Personally the position has nothing to do with it as whether he's a LW/RW or C he's going to dominate the OHL again at 18. There's no need to re-invent the wheel in terms of development by thinking he can somehow learn more about being a C in the OHL vs a full season with the Leafs with a Marlie conditioning stint or 2 to train with pros and learn from pro coaches.
I really think you all spend wayyyy too much time obsessing over the positions of our forwards.