Teams pay a surplus in FA because they are bidding against other teams. UFA's have no assets to bargain with so they are not part of the negotiation. If only one team is interested in a player in FA there will be no surplus.
Marner doesn't hold all the cards. On the face of it, he should have the upper hand, but even that's not necessarily true. These situations in real time are more nuanced than you describe them.
The Leafs are in full control of Marner for the duration of his contract so long as he is a Maple Leaf. They also hold the hammer in any three party transaction. Thats not nothing, and can be exploited.
Applying game theory, if the Leafs want Marner out, his best path is likely through cooperation.