How do we know error from correct version, we don't. What if Oliver made the error first when writing it down and then Joseph corrected it before print versus Oliver writing it down correctly and the printer then made the error. We'll never know.
I think everyone is in agreement that polygamy was practiced culturally, that almost goes without saying. The prohibition is on the spiritual side and it comes from the Lord categorically disavowing that practice in its entirety in Jacob 2.