As some here will know. I dislike 'grey areas'. My experience leads me to believe that such areas are often inhabited by people who seem to 'know better' than the rules allow.
Not normally a situation which enhances safety.
Some of the a/c being discussed, are reasonably likely to be fully equipped and regulated for two pilot ops, though at times flown single pilot. That's fair enough.
The point is, if the second pilot has a type rating, he may use the radios or touch controls, as long as the Ops manual caters for this for a public transport flight. If not, he cannot do either because yes, he is classed as a passenger.
One assumes that each pilot also requires a specific qualification, and sim check to sit in either seat? So for example a captain would need a specific LHS qualification to be a 'co-pilot' on a PT flight? Also some form of MCC qualification might be required from BOTH pilots for PT? Otherwise one presumes they are no more than a bag carrier/map folder, and regarded as completely unqualified to play any part in the operation? Which is precisely why they can't enter such time in their logbook.
Other than on an instructional flight, of course on a helicopter laid out for just a single pilot, any one in the LHS cannot possibly be anything other than a passenger, and certainly not on a PT flight. Period!
A millionaire paying £500 for a 'second pilot' could be paying £500 just to carry one extra passenger! Rather naive?
Edited to add:
Shy, good link!