This would be quite complex if it came to court.
You are correct to say that a radio operator can be part of the flight crew designated as required by the pilot in command but there are a number of related issues that could cause you problems;
1. If you have your own radio licence and thus a radio operator was not required, it could be argued that you were simply avoiding the insurance requirement.
2. The estate of your deceased radio operator will sue your ass off because they do not see the joy of flying and avoiding insurance in the 20/20 hindsight of a nasty accident.
3. You will have to provide some insurance for the crew including yourself and the other crew member and the insurance company will ask very searching questions. No insurance - reread point 2.
The only time that I have come across large crews in something like a C172 is when avoiding airport passenger charges - Was that 1 crew and 3 passengers sir? - NO Mr. Airport official, It is 4 crew - pilot, radio operator, navigator and the navigator's instructor here are their licenses..................round to the duty free and then customs..........Was that 4 crew sir?- NO Mr Customs officer it is one crew and 3 pasengers and here is their full duty free allowances and my 1/2 allowance purchased.
What is the problem you are having with insurance - price or not able to find anyone willing to provide it?
Regards,
DFC