Well mature adults are able to sit down explain their financial position, team needs and future plans going forward. If it doesn't align with Marners vision for the future then at that point you have to ask him if he'd waive, and where he would like to go.
However, not under any circumstances do you take the negotiations to the media. They made that mistake with Sundin, and it caused a lot of hard feelings. The Leafs thought they could control the narrative and tried to demonize Sundin and making it uncomfortable for him to stay.
Sundin was a good soldier while wearing that C for 11 years and they treated him shabbily, instead of doing the right thing with him.
I'm sure if Mitch really wants to stay he may be a little more reasonable in negotiations, how much, who knows. Maybe offer Mitch a 12m 2 yr deal and when the caps at 105+ if theres been some playoff success, especially on Mitch' part then you make him a lifetime Leaf.
I'd say do the same thing with JT. Also by that time you'd know what you really have in Minten Danford, Chadwick, Grebenkin, Holinka, Webber and Cowan.