Let's parse that out into the exploits you want to have occur:
Player A and B are at war, and in sequential turns.
Player C wants OB with Player B, so that he can STEP IN FRONT OF Player As army, thereby preventing Player A from doing anything with their units during their turn.
When it's Player B's time, Player C (who is still in simultaneous turns with both Player A and B) moves his units, which allows Player B to attack Player A without reprisal.
as per civvies - GG's are also 'civilian' and so if you could move a GG/Khan around during Player As turn to have the GG/Khan be supporting units that Player A is attacking at that moment, then you are gaming up the turn sequence.
People complained for a long time that they wanted multiplayer sequential turns, so as to prevent spam click style games, and now you complain that it's there?