I think the Leafs will just run Marner back without an extension, to give him another chance, figure out if they can "replace" him with Easton Cowan who is a good Junior player right now. They know they'll lose any Trade, especially since he'd just pick his destination, and just get worse next season without their draft pick (top 10 protected, but they wouldn't dip that far). Also other perspective is that it's not a situation where he's demanding out and saying he won't extend, he'd probably sign an extension at a rate that works for him, so the Leafs aren't in a no win situation there and can assess their options.
Everyone in Toronto is mad right now. They'll settle down in a month, talk themselves into a new Coach might make the difference and then just more or less run it back again. The one guy they might try and boot out the door a year early is Tavares.