Incorrect, Nylander never complained when he was put on the 4th line.
There were rumors Marner had asked for a trade twice when he got demoted. This explains him being unreasonable in negotiations. He also wasn't happy about not getting rookie bonuses.
Rumours? Is that the base of your argument? Also how is he being unreasonable in negotiations? The Leafs are in cap hell right now, and can't afford to pay him what he deserves given the current cap ceiling they're dealing with. Dubas basically traded Sparks for a $750k break with the cap, in order to protect against any incoming offer sheets. Mess already explained how the cap works in another thread, which also explains why Marner's not signed right now. He potted 94 points, and plays in all situations. Players like that don't grow on trees. The team would have to shed over $10m in cap space before even the LTIR kicks in, to even give Mitch a decent 1 year offer before the season starts.