What could we get for him and do any of these work

That's a pretty fair MN trade IMO, although we really have no reason to move Dumba (or Greenway for that matter) which means you'd have to overpay (aka a 1 for 1 Dumba for Marner). Obviously that's dumb for the Leafs, but any trade they made involving Marner is dumb. Kid should be a career Leaf, it's incredible the amount of talent he has.

Of course he is going to be a lifelong leaf but this is about finding out what we could get for him

I would just like to note that this user in no way represents MLSE and yeah I bet that trade seems fair lol...

Mariner is a proven product
Nylander just signed
Isn't the better move Kapanen/Johnsson and some of your debt (Horton, zaitsev)?
Kadri has a ntc
Rip the band aid off
You have great assets in mitch Mariner etc, terrible problems to have to deal with

I think I could be swayed on Ekblad or Parayko, but it would be an agonizing decision