Let's not conflate employment terms ( full time, part time, short term, long term, etc....) with service providers. As offered by the OP, they want to outsource internal audit to an outside vendor. Nowhere it was mentioned that they want to employ someone part-time as an internal auditor. BTW, many aerospace CB's offer to perform first party audits for organizations they are not involved with, in terms of certification. Obviously CB's are accredited and that would answer one of the OP's first questions in this thread.