Let me see if I understand correctly. You're saying that:
1. Lea's deal with Harry was contingent upon Justin's death (permanent death).
2. Lea selling Harry's debt to Mab gives Mab power in a real, magical-oomph-type way.
3. If Harry sees that Justin is alive, that negates the deal, and Mab suddenly loses the power she had.
If I understand this correctly, I think there might be something there. I'm not sure that obligation is a direct power boost for the Fae, so I don't think it would necessarily shake out precisely that way, but it might cause problems for her if Justin comes back.
I believe, though, that Lea's deal with Harry was specifically for the power to beat Justin, not "Justin will die." Ghost Story does not give the specific wording, though, so I could be wrong. Even if that was the case, Fae Rules-Lawyering would say that Justin's heart stopped, which killed him, and any event that brought him back is not their concern.
But if there is sufficient wiggle room in the original contract, I do believe that Harry could use that to negotiate with Mab—perhaps to negate his remaining favor while letting Harry leave the Winter Knight's mantle behind. (Though I don't think he needs to; just figure out how to drop it and wait till Halloween to pull it off).
I do want Justin to come back, if only to provide some insight into Harry's past. Maybe in Mirror, Mirror, if Cowl pulled off the Darkhallow, Justin forced his way back across the Veil in the necromantic turbulence. Who knows?