I'll answer your question, but I would like to also address a few things first.

For one thing, Wyrmbane's only appearance in Cataclysm so far is a brief cameo in a Horde-side quest concerning the 7th Legion's work in Gilneas.

Secondly, Agmar is only an Overlord in the Kor'kron. High Commander Wyrmbane is the leader of the 7th Legion. Wyrmbane's counterpart is actually High Overlord Saurfang, and he opted to stay in Northrend with a small detachment to secure their holdings in the Borean Tundra.

Thirdly, the conflict in Northrend was between the forces of Azeroth against the Lich King - not Alliance vs. Horde. Despite the fact there was much in-fighting between the two forces, both sides were fighting for the same thing: to overthrow the Lich King. That was the main purpose for both sides going to Northrend.

To answer your question directly: No. The Kor'kron did not "lose" the conflict in Northrend. Both the Alliance and the Horde won, for once.

No, not that conflict, I'm referring to the isle of conquest.

In the isle of conquest, Overlord Agmar's forces and Wyrmbane's go head to head over control of the island. Wyrmbane appears in Cata, and no matter how insignificant his appearence, it means he's still alive. Agmar doesn't, he might have died on the isle of conquest.

Can't make conclusions like that. For example, both Vanndar Stormpike and Drek'Thar are in the cataclysm question chain through hillsbrad. As are Captains Galvangar and Balinda.

Probably have to ask an RP'er for some kind of advice on how to explain all that by some lore; one of those things that most people never think of I'd gather. AV and IOC are the only ones with PvE end bosses (unless there is another that has completely slipped my mind).

Saurfang retire!
The only way you "retire" from the Horde is by death on the battlefield!