It is possible for a councillor to act as RFO, although undesirable except as a temporary expedient. They may not receive payment while a councillor and for twelve months after ceasing to be a councillor.
The internal auditor must be independent of the council. If you are saying that the RFO and internal auditor are one and the same, that is totally unacceptable. If there is no effective internal auditor, you would be entitled to make a complaint to the external auditor, and there would be a real risk of the council failing its audit if it did not institute proper control procedures.