... That might be their plan, but I doubt that it will be that simple or that Mab will be fooled by it ...
Of course it won't be easy, and of course it won't fool Mab!
But IMO Mab doesn't want the marriage to go through: she wants things that she expects to happen during the engagement.
So long as they show no overt disrespect to the Queen of Air and Darkness, Mab will be 100% satisfied with them not getting married.
I'm sure she has contingencies for if the marriage
does take place, too.
... It may be another of her not all that transparent plans to take either or both Harry and Lara down at least a peg. You can bet that Mab has a plan, and it isn't what she claims, a stronger alliance between the Winter Court and the White Court.
I'm pretty sure that Mab is pretty sure that there is some locus of Nemfection in the White Court, and that she intends to hit it with a "patented Harry Dresden anarchygasm" boosted by Winterknight steroids.
Incidentally, she forces Harry (yet again) to operate with an uncomfortable ally, to do diplomatic & social things, etc -- weaknesses of Harry's, that Mab is working to strengthen.
Also, Papa Raith has a substantial library around the Starborn cycle & current iteration of the apocalyptic Outsider invasion, and I'm sure Mab has more than an inkling of that... and that getting access to it for her Starborn Winterknight will substantially bolster his Outsider-fighting ability.
If they
do get married, I expect the consummation to turn violent (and Harry will kill Lara): WoJ has stated that Whampire-Mojo and the Winterknight-mantle interact in explosive and uncontrollable ways.
Mab will put success on
any of these fronts into her "Win" column, and I 110% expect that she has other agendas I haven't spotted.