Backpacker,
You need to differentiate between the person who manipllates the controls, and completes various tasks associated with the flight and the person who is responsible for the safe execution of that flight.
I will say it again - The pilot in command is the person who is responsible for the safe conduct of the flight. Note means flight from start to finish.
The person who does the flying may not be the pilot in command because while they do the actions, they are not responsible for the outcome of the flight.........i.e. if something illegal is done then it is the pilot in command that will be held responsible.
The simplest way to determine the pilot in command is to ask yourself.....if there was a flight plan form filed for the flight who would place their name in the pilot in command box.
Note that like everything else, the flight plan does not provide for a change in pilot in command.
You are correct to say that if two PPLs depart then the pilot in command must pay all or an appropriate portion of the costs.
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Bose,
Refer to the above in the case where the aircraft is not signed for.
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Of course the real problem with a non-instructor completing a 28 day check is what happens if the pilot being checked is not up to standard. The PPL is not qualifiued to decide that and also they are not qualified to provide instruction to bring the pilot up to the required standard. Taking that into consideration, it is hard to see where such a check completed by a PPL would ever stand up in court..........hence one would have to say that unless one uses an instructor it is pointless having such a check.
Regards,
DFC