QUOTE---In reality it is very simple. A single crew a/c can only have a single pilot (unless there is an FI giving training onboard, but even then, there is only one captain. For example I cannot log the time that I send a student solo, though I do get paid for it.) so in most circumstances, only one pilot can log the time, basically, the person who signed for the a/c and who in the event of a disaster will be deemed PIC in absentia. You sign for it, you log it. It gets broken, you answer for it, not the "passenger."
Its no wonder your shaking your head, your tying youself up in knots.
Lets take your example, you say a single crew a/c can only have a single pilot.
Let me give you two examples of how your misunderstanding of what you are talking about isnt quite correct.
1. On the same night that Graham Hill was killed in around 1972 Eric Auchin and Arthur Penzer were killed at Birmingham making an approach and go-around in 100 metres visibilty ( in fact 3 approaches.) There were flying Eric's Baron which according to your logic is a single pilot only aircraft. Eric Auchin had a medical condition which meant he could only fly with another qualified pilot. There were two pilots operating this aircraft. The subsequent AIB enquiry could not establish who was handling the controls at the time of impact but evidence from RT transmissions and other pilots who knew the pair well lead the enquiry to believe that the much more experienced Penzer had taken over to fly all three approaches from the right hand seat and that this had been a posible contributory factor in the loss of control during the go-around.Auchin had booked out as P1 with Penzer shown as the passenger. Auchin was however not qualified to fly IFR in controlled airspace so ask yourself who became the commander in this incident.
2. Another incident where the pilot of a light aircraft, which you call a single pilot only a/c, was flying with a friend who was also a qualified pilot. during the flight the commander had a heart attack and was incapacitated. Subsequently the passenger took control and became the commander and landed the aircraft safely.(wonder how they logged that one then!)
In both of these incidents these two aircraft, both minimum single pilot aircraft, had two pilots.
I should add also that one of our Captains (who has recently retired) had a heart condition and while flying our aircraft he could only fly with another training Captain(and yes under JARS) if he had wanted to fly a light aircraft he was in the same position as Eric Auchin in that he had to have a qualified pilot beside him.
In regard to Log books the entries are evidence of your experience, the time you spend handling the controls is not the issue. As commander, and the phrase captain was dropped many years ago, you may be exercising the privleges of your licence with minimum or nil contact with the contols. Your log book entries for the purpose of the order shows the time you acted in the capacity of commander or pilot under instruction or pilot other than commander.
the fact that you book out as commander dsont not mean that you will land as such!
Many years ago the commander of an aircraft wishing to fly along the Berlin Corridor had to sit on the flight deck as an observer(passenger) of an aircraft and fly along the corridor with someone else in order to get the nesecarry consent--guess how this was logged in my log book and I didnt feel the need to buy another log book!?
So you see in reality (quote) it is not very simple-- legislation is always a matter of interpretation--interpretaion is based on experience and those with the widest experience can usualy provider wider interpretation!