I would say the previous owner was responsible for not providing the proper humidity. The snake starting a bad shed the first week the OP owned it can not be blamed on the OP. It would take longer than 5 days to cause a bad shed and longer than 14 to fix it.
As far as misting causing an RI, that is news to me. I mist many of my snakes daily and during a shed sometimes more and have never had an issue.