You will loose buoyancy if no o2 is added and you are metabolising whats in the bag but to notice it you will have to maintain a perfect constant depth, pretty hard to do in open water (may be better in a cave) Far better to watch your po2 displays!

Its better to use your mouth as you can get rid of gas without having to breathe it first, everything that comes out of your nose must first be breathed in. I am not saying you cant use your nose, in my opinion its easier to use your mouth

Dave