Hi van, You're correct on a/. Coordinate and cooperate are used intransitively here. Coordinate also has a transitive use, but cooperate does not.
Your phrases in b/ are not used in the sense you intended. "Tenure" comes to mind, although it's often associated with college professors to describe a very secure status of employment, from which they can be fired only under extreme circumstances.
"During his tenure with the company he has made many contributions, leading the director to raise his salary."
Edit. My dictionary and my thesaurus don't seem to support me on "tenure" for general use as a period of employment. It's rather like holding office.
I think I'm skunked for now. If I find something, I'll come back. - A. (Maybe, "While in our employ, he has made significant contributions to the company.")