Insurance companies in my experience do not procrastinate on how/who/when a pilot is the PIC. They keep it simple and insure in accordance with the national authority regulations. They will of course sometimes stipulate restrictions, such as experience on type, but these will be clearly set out in the policy. They normally grant exemptions when asked to insure a safety situation say: when a type is single pilot but there is only one set of controls. This may include a CRI/instructor or another approved person to be in the non flying seat during a conversion, which most underwriters encourage.
In multi-crew aeroplanes regulation makes it clear who/when a pilot is the PIC but this is not comparable to single pilot operations, whatever the view of barrack room lawyers. For me it is a complete no-no for the PIC to ride in the rear seats with two others sat at the controls and it can never be justified..