You may have another OPTION..
Water closets TOILET also come in various roughings have you considered a 10″ rough as opposed to a 12″ or 14″ rough?
Why would any plumber want to use plastic for drainage given a choice?
Oh well it is no supprise that the measurements were off considering the materials that were used.
The problem with offset floor flanges is the real possibility of causing a slight restriction during dischagre of the fixture if it is not set properly.