Why? Xanatos is perfectly content to maintain status quo with both the Clan and Oberon's Court by the end of the series. He's actually more stable than Marcone. In addition, Alexander is the grandson of a version of Titiana herself and possess tremendous magical potential. If anything, Mab would salivate at the possibility of having her great-grandson, taught by no one less than Puck himself, at the head of Marcone's territory.
And since Mab can command a certain security head in his Jolly Saint Nick persona (also revealed in Skin Game), Marcone is more likely to find himself without mystical allies or protection of any sort if he tries to negotiate with the Fae for assitance because Mab would gain more with Marcone gone and Xanatos in his place.
Sorry man, Marcone's boned in so many ways in this fight.