I doubt Nicholas would have joined the allied forces. He changed a lot after the resurrection ritual.
Each of the four who opposed Kha-Beleth had their own motives. Godric's goal was to restore the Queen of the Empire to her rightful throne, Railag's goal was to save Isabel at all costs, and Faidaen and Zehir's goal was to prevent the Dark Messiah from appearing.
It is unlikely that the vampire Nicholas cared about his former fiancée or the fate of Ashan. Perhaps, after the resurrection, he even hated Isabel. After all, because of her, he was cursed, because of her, he left Elrath forever. Therefore, IMHO, the return of the queen to the throne would hardly please Nicholas.
Added 3 minutes ago
...So to speak, he would have gone through a kind of purgatory
Added 1 hour 32 minutes ago
Fanatic and Diplomat)) The most curious thing is that he has the combat skill "Tactics" in his skills) not to mention Attack...
Added 3 minutes ago
He clearly wasn't going to join anyone) The Necromancer who resurrected him didn't do a very good job. Or, to put it more accurately: he resurrected him "badly," sorry for the details))