As revealed in the next episode, he escaped. He was not a traitor, he just got lucky and was outside at the time.
It was just a small force he brought with him. It seems to be more than 12 yes, but also less than, say, 100. Tywin didn’t know the exact number, he was very far removed from the action. Maybe 12 was his best guess. Anyway, 10.000 or 80 at dinner hardly alters his point.
It was a pretty good plan; marry the ginger bird into your family first - get her onside - and then offski her eldest brother. Finally, hunt down the two boys and job done. Without a war.