2. I'm not sure what you have in mind for "short term" vs "long term", but oxygen levels only change significantly over hundreds of millions of years.
3. Just like a stable sized forest doesn't produce any significant net oxygen, humans don't overall use up oxygen from breathing. The plants you eat (or if you're eating meat, the plants your food ate) released oxygen into the air when forming the carbohydrates and fats you eat. Your body then recombines these with an equal amount of oxygen to produce energy.