Landeskog is not a centre. Just because someone can play centre, doesn't mean they are one. & just winning faceoffs alone doesn't qualify someone as a good centre. Landeskog played on the wing of MacKinnon who isn't the greatest face off centreman, and there was many many times where he was waived from the circle and Landeskog would fill in for him. With MacKinnon also being the guy who runs the PP, he usually isn't the centre on the man advantage and Landeskog could step in there. But he's by far not a 1C and not a consistent centre option at all.

leafs need to get a D man off their roster in order to take another guy, dermott or holl would have to leave

No they don't. No team goes through a full season with only 6 defenceman, depth is key. They'll likely keep them because their depth in the AHL isn't ready yet

No need to get all worked up over it because It really doesn't matter Landeskog isn't signing in Detroit anyways.