Your illustration shows the warmest way to wear a mummy, but i am always twisting and turning and thus breathing into the bag, etc., and it gets to be quite a hassle. If it isn't bitterly cold,I like to cinch the bag around my neck and put on separate insulation around my head and neck. A balaclava works great for that. Then I can flop around all I want.

No one has mentiond that sleeping in a group makes a difference, such as being the middle person in a three person tent.Mild exercise will warm you up if things get really cold....