Glad you were able to find the problem, I never would have been able to help you remotely and a 10/2, 2-story roof is out to the question for me. That is a really weird problem in that the system has been working fine until now. Sounds like the screw up on installation could have been somewhat fatal and yes, your makeup/combustion air may have been comprimised.
However, you now have some spare parts being that the old pressure switch was most likely not defective. The only advice I would provide you is that you invest in a CO2 detector for your home. It's cheap insurance just like a smoke detector. Keep in mind that CO2 dectors do have expriation dates.
I hope he gets it fixed ASAP, we're supposed to get dumped on with snow for the next 2 days and going onto the roof will be out of the question for him too.