The premier war leaders in the North are Henselt, John Natalis, and Iorveth.
I think we can rule out Iorveth coming to the aid of the kingdoms; if Upper Aedirn is still in dispute, Iorveth and Saskia will lead a guerrilla war there and make life miserable for at least one of the remaining powers.
I don't trust Henselt any further than I could throw him, which wouldn't be very far at all. Although he's an able commander with considerable resources (even though depleted, and the Dun Banner will be sorely missed), he's also the most likely to sell out, if he hasn't already.
John Natalis's position is, unfortunately, precarious. He still commands a considerable power, but how long it will remain together is an open question. The money to pay and equip an army has to come from somewhere, and he is already heavily in debt. The Temerian nobles will render no aid; they will demand too much for empty promises. I think he has to fight for a quick victory or a miracle. If anybody could pull it off, he could, but his task may be impossible.
Radovid is a cunning little pup, but has not the battle experience to promote confidence in him. But he reminds me a lot of Henry VII, and Henry turned out to be both an able commander and a ruthless politician who could destroy his enemies without having to lead an army against them. He has two of the strongest armies: his own, and the Order of the Flaming Rose, at his command. So he is to be reckoned with irrespective of his lack of field command experience.