Usually with nitrogen, time instead of depth is the limiting factor for a dive. Whereas, with oxygen, depth instead of time is the limiting factor. The Nitrox class will show you how to calculate specific oxygen limits. Nitrox itself is not likely to affect your air consumption, however, NDL are longer. There is no definitive answer to this question.
More study is required.
The nitrogen partial pressure is governed by your EAD which will be shallower than your actual depth. Yes, although Nitrox greatly reduces the chance of getting 'the bends', DCS, you still have to watch your depths and times. The Nitrox class will show you how to calculate your depths and bottom times. The Nitrox course is only about 4 hours. And the math consists of only 2 formulas.