I don't know that anyone has the real answers to your questions. I think part of the answer lies in how someone has always done it, and what they are comfortable with, and that's why some of the CRNA's you meet are resistant to lower O2 concentration. Here's my thoughts.
We know pure O2, over prolonged periods of time, can cause pulmonary damage, probably though free radicals. By prolonged, that generally means (as I recall) more than 20 hours breathing pure O2. So, for the course of most surgeries, running a patient on pure O2 won't be harmful, because they are going to be there for less time than it would take for O2 to start causing damage.
Generally, I adjust my gas flows based on a number of different factors. If the surgery is going to be very short, I don't generally mess with adjusting flows and leave the patient on 100% O2, for the reasons mentioned above. Also, if the patient has a cardiac history, I'll generally leave them on 100% O2, just to get the maximum O2 to the myocardium.
For longer surgeries, and healthier patients, I will generally adjust my gas flows so I am delivering about 50% O2 to the patient. Other than that, I will adjust as the SaO2 directs. When I went to school, there was a real "anti-nitrous" sentiment, because there was some tenuous link between N2O and postoperative nausea. I almost never used N2O. Now, I have worked with N2O a bit more, and am more comfortable using it. Its great for use when you want something of a faster wake up (nothing like waiting for a patient asleep on forane to wake up after a 4 hour surgery). It's also great for increasing the depth of anesthesia in a rapidly reversible fashion. Again, I adjust to about 50% O2 flow. I have noticed no greater incidence of postoperative nausea in the patients I have used N2O in over other patients. I think the studies that suggested a link between N2O and nausea did not adequately account for other factors.
Kevin McHugh, CRNA