Happy Bird: When I did flights like yours ie. augmented crew due to flight time limitations issues then the captain is the ONLY crew member who logs the entire block time for the flight. He is considered, in the eyes of the law, to be "in command at all times" even when not on the flight deck!
This is a hangover from maritime law.
The other co-pilots log the actual time that they operate from their designated seat. (this may be LHS or RHS depending on company policy regarding training from said seats) Thus the other crew would normally log about one half of the block time. I have heard that certain ME carriers even exclude the "resting hours" from the annual 900 hrs total - although I have no proof of this.
With regard to IFR/IMC Do not confuse IFR (how the flight is conducted ie. rules) with IMC which is actual flight conditions. For example a flight may be totally IFR but completed entirely in VFR conditions.