Your bivy sack touching your sleeping bag will help it to move through to the outside. Most bivy sacks do touch.
On the dew thing. Your sleeping bag insulation gets progressively less warm as the water vapor moves from your body, through the insulation, to the outside of the bag. When it hits the point where the vapor cools down enough to turn back into a liquid, it condenses. Whatever layer it is at in your bag it stays there. As things cool down through the night the depth of "cold" can increase to closer to your body. The liquid water that was stuck there now gets cooled down to the freezing point and you get frost. This most often occurs on the shell of the bag or the inside of the bivy sack because it is place that is finally cold enough. In cases of extreme cold though it can happen in your insulation in your bag.
