Happy Bird: I think you are over-complicating this issue!
a) take off and landing times will define the total flight time. If that is an issue for you - draw an extra column in your logbook for it.
b)any time spent taking rest/off duty in flight cannot be counted for licencing purposes except for the captain, as I explained.
c) if your company requires you all 4 crew to be in the flight deck for climb and descent - then this time counts as P2 unless it is P1CUS as listed before.
All of the licencing authorities are well aware of crewing arrangements for heavy crewed flights therefore it is important that you do not try to log your "bunk time" as operating time!
When I was operating as relief crew I did many flights of up to 14 hours where only 6 or 7 hours were logged, and when operating as a single relief pilot giving relief to the two other pilots I would log MORE operating time than the pilots who conducted the take-off and landing!
On a B747-200 with a 5 man crew, the relief would do 10 hours in the seat to give each other pilot 5 hours off duty time during flight. Now that was tiring!