I don't think there's any argument that Treliving can make that Mitch should make less than Willie's $11.45. Outside of goal scoring, Marner's contribution to the team is unquestionably more than Nylander's.
So unless Mitch is feeling charitable, which there is zero indication that he has been or will be, he's getting full value. Which likely means the negotiations don't even really get going until MLSE acknowledges the starting point for talks is north of, or very, very close to $13M.
The only question then is if MLSE will follow the Av's philosophy (for now at least) that no one is getting paid more than their 1C. I doubt it but if that is the case and Mitch wants more then he's likely not going to be a Maple Leaf on July 1st.
My almost certainly way-off prediction is that the deal gets done just before July 1st (perhaps mere hours before), coming in at something like $13.1M, just under Matthews.