The division Paul spoke of was not about things like what title to call Mary by. The divisions Paul spoke against was that who to follow. Some were saying Paul, others were saying Appolo, others were saying Christ. That is what Paul was upset by. People are saved by belief in Christ Jesus, not by any church or its numerous doctrines.
What to call Mary by is indeed unimportant, unless, like the objectin to the “Mother of God”, it betrays a christological disagreement. Our marian devotions differ, for example, in the West and in the East. I was always puzzled by the vehemence with which Roman Catholic expressions of piety are attacked by the Protestants.
The true problem is indeed whom to follow: the followers of Luther, and, especially, Calvin simply don’t have the Catohlic belief system intact, starting with its core, the Eucharist. We don’t have visible communion because we don’t have the sacramental union.