getting the job initially is the same as any other federal job: have a college degree, some experience in the medical manufacturing field, and doing well on the interviews.
passing the certification process is a little tougher. you observe a couple audits, then perform some joint audits, then you perform an audit by yourself with an inspector evaluating your work. its not a formality, some people make it, some have to go back to doing simpler audits such as foods or drugs.
to become an expert means you have to be an expert in the law. in particular the Food, Drug, and Cosmetic act. i don't think being a CQA, CQE, or SSBB will help that much. medical device experts have a lot of experience in the agency (20+ yrs) and are usually called on for assisting in possible criminal investigations or to explain the FDA's position in court.