"1. Lets say that I just came over a structure and I want to get back down to the bottom (10ft or so). Would there be any possible harm from exhaling all or some of my air and holding that until I reach my destination below? "
You can exhale and inhale to change depth unless you are using a rebreather, I do it all the time, the trick is to keep the airway open so you are not really "holding your breath", just controlling lung volume. Shallow breathing to sink, big breaths without exhaling all of the way to go up. Stop breathing when taking pictures, don’t breath if I’m taking your picture :)
Never ever close down your throat except to clear your ears.