Let's see if we can agree on one thing: It is open to me, during an AFR, to terminate the review and take over as PIC from that point
I would say you absolutely can't do that, or "PIC" has no meaning. I think you'll find the "flight" is defined from start to stop, and thus the PIC is for that flight. Once the PIC is designated for the flight, that's it until you land - regardless of if the owner is on board or not. It's irrevelant if the owner happens to have a licence.