I think Mab does need a marriage to cement this alliance. Bonds to kin are extremely important in Dresden Files, not only that, but alliances by mere agreements or concessions can but subverted through treachery. The type of treachery that a lone or subverted individual might initiate to implicate an entire organization ala The White Court's financial dealings tied to La Fortier's murder, which Mai views suspiciously. This despite both factions being in a cease-fire or state of neutrality.
A marriage however? That reduces any form of treachery as work of a traitor or lone wolf by the very nature of both Courts being tied as kin. Any attack against kin is viewed as an attack against one's own self interest. Especially a marriage between two members of the Accords where old school rules and obligations apply. Only someone acting *against* the will, and self interest of their respective organizations would dare do that. An example of this is shown in White Night, where despite the stand down order of the White King, two opposing internal factions disobey those orders under pretense of acting in the best interest of the entire White Court, thus giving them a pass until Dresden calls them out. A marriage between factions closes that loophole.
