No, let's say that Axel, Larxene, Sora, and Riku Replica all side with Marluxia. Now with Zexion, Lexeaus, and Vexen being killed at castle oblivion. So Sora and Riku Replica team up and whoop Xaldin and Xigbar. Axel and Roxas whoop Saix and Larxene handles Luxord. (Roxas and Xion would probably side with Marluxia if Axel did plus Demyx would out of fear.) Now while they take care of the members loyal to Xemnas, Marluxia takes on Xemnas himself.
There's no way Marluxia could take on Xemnas alone. It took two teenage boys to kill Xemnas, Marly has no chance. Also, Roxas would disappear as soon as Sora shows up.