(European answer) It's amazing how complex people can make this. I follow this rule:
- Designated aircraft commander (nominated by company, who signs the tech log) - P1
- Second in command (irrespective of rank) - P2
Pilot flying is simply pilot flying. He doesn't assume command just because he's handling the controls. Similarly the commander doesn't cease to be P1 just because somebody else is handling the controls. It has no bearing on who is P1 or P2.
The seat being occupied is irrelevant.
PICUS need only be used on the (comparatively rare) occasion that the P2 is for whatever reason acting as though he/she were the commander without intervention on the grounds of safety, be it for command upgrade training OR (certainly in Europe) as a 'work around' to help co-pilots without the requisite PIC time for ATPL issue get their PIC hours up.
Hence your situations are impossible to answer as there is missing information; but in general terms in all of your situations - the legal commander is P1, the other pilot is P2. If the flight is part of a course of training in which the second-in-command is for whatever reason acting as P1 without the intervention of the commander on the grounds of safety, the roles are P1 and PICUS.
To emphasise the point, in scenario 1 it could be EITHER that the LHS Capt is P1 and the RHS LTC is P2 (right-seat qualified, called off standby as no co-pilots available maybe?), alternatively, it could be that the RHS LTC is designated commander (and hence P1) and the LHS Captain is PICUS as part of some specific training, OR depending on what is allowable in the operator's Ops Manual it could be that the RHS LTC is P1 and the LHS Capt is simply acting as P2 (rare, but not impossible).