Nah, as I said, the marriage won't last because nothing last for Harry. But to cover the alliance, Thomas becomes the WK. That puts a Raith in service to Mab, tying the WiC and WhC together through Thomas. The marriage is one of Mab's ways to pay off the debt, but not the only way.
This is if they even make it down the aisle to get married, which even that may not happen.
And I can't see it not happening.
I don't think Jim is going to give us the cure for Thomas and Harry's ascension to a new mantle in the course of the next book, especially one that wasn't part of the original plan.
Plus for anyone to become the Winter Knight, they'd either have to kill Harry or come up with some new way of passing mantles that's not Halloween because we know that Harry still has the mantle at Christmas. I can't see Harry getting another powerup before the BAT begins and maybe not until the second book of that even.
And Mab wants Harry as Winter Knight because he's her chosen weapon and he understands the duty his power and her entails since his experience with the mantle. Harry may rail against it but this wedding isn't a big enough issue to break with Mab over or he'd have done it at the end of BG.
But Mab did give Harry a year to make things different, so all he has to do is come up with something that gives a stronger tie between the courts publicly and privately than a joining of bloodlines entails and that Mab and Lara will go along with.