Defending is automatic and does not require a red cube. So your Bayonette guy can complete his attack move.
When the bayonette guy kills the unit in the bunker he then is in control of the bunker.
I'm not as good as the designer but this is my best answer.
I've read the rules a few times, sorry if I missed an obvious answer to this question but I am a little confused about the bayonette special action. I know that a unit with this ability can be given a green and red cube to do an engage action ability and instantly defeat an opponent's infantry unit for a base cost of 2 skill (provided it survives the defender's attack). How does it work in this scenario:
My opponent has an infantry unit in a bunker in zone A. I move a unit with the Bayonette special action from my zone B into that area with a green and red cube.
A) Supposing my unit survives any suppressing fire, can it use its Engage action ability to bayonette attack the unit in the bunker?
B) If it can bayonette the unit in the bunker, does it then gain control of the bunker or does it fall back to my zone B in the spent unit section?
Thanks in advance for any clarification about this.