Can an employee in LEED consultant firm who was not involved in the project be the CxA ?

As long as the LEED consultant is serving no other purpose other than guiding the submission process, assisting in the documentation, reviewing documents for compliance, etc. (as you said, no design responsibility at all), then yes, this is acceptable. This has been covered in past interpretations. The role of the LEED consultant is definitely a part of the design process, and that is what worries some people, but typically the role of the consultant does not "direct" the design and has a communication path directly to the owner as well as the design team.