If a unit is declared a defender or attacker and before combat resolves I prevent him from being an elgible defender, can he still defend? For example. My opponent declares a defende and I use monster of the deep's effect in response. Is he still a legal defender or does my response nullfiy his declaration?

Yeap, a unit, that was already declared as an attacker/defender and then was corrupted still participates in combat.

Okay cool. What about if a unit gets bounced back to your hand instead? Then does the defender still participate in combat? Because he isn't even in play anymore?

Of course not