Usually when a veneer is done it only takes 10-14 working days to come back from the lab where it is made. I don't understand why you had temporaries on so long.
Nitrous oxide is very safe is used properly. Here in Ontario, Canada only a dentist can administer the nitrous and the patient should never be left alone.
I agree with Scottish2008, why would you go back to the same dentist after having restorations fail? Any dentist can be in a magazine, they often PAY to have articles and/or advertising put in magazines, doesn't always mean they are the best in the area.
I am sure that Dallas has many better qualified dentists than the one you've seen. I'd suggest letting the State Board of Dental examiners.
Texas State Board of Dental Examiners - Complaint Process Explained