Does he have to be attacking and a unit declared a defender for his 'opposed in combat' 1 uncancellable damage to occur? Or does it work when he defends!? I guess he would be the opposer, anyway - an easy answer for someone who knows.
Opposed in combar Friedrich Hemmler
Yes and yes. "Opposed in combat" means attacking while a defender is present, or defending while an attacker is present. This effect will trigger off of blockers being declared, regardless of which side he is on.