So let me get this straight. You've been happy to take-off, fly IFR, and approach (potentially) to IFR minima with no agreement between the two of you as to who is PIC? Astonishing!
What about captain's briefs before departure and approach? As someone putting themselves forward as a 'safety pilot', do you not consider those to be an essential aspect of IFR operation of a multi-engine aeroplane?
I would suggest this conversation is one that should be urgently directed to your 'client' rather than undertaken academically on PPRuNe!