Theotokos (Greek) = God-bearer or Mother of God
The Church at the Council of Ephesus in 431 A.D. declared Mary to be “Theotokos,” the “God-bearer” or “Mother of God.” This was to clarify that Jesus was the Second Person of the Trinity and not two distinct persons (the Nestorian heresy). The doctrine had its origins in defining the nature of Jesus (one person with two distinct natures - human and divine), in order to combat this heresy. By saying that Mary is the "Mother of God," one acknowledges that Jesus, the Second Person of the Trinity, was born from Mary's womb.
If Calvin didn't want to "harden the ignorant in their superstitions," then he should have explained what the term meant. The fact that he allowed the "ignorant" to remain "ignorant" shows that he was either ignorant himself or he purposefully didn't explain the correct meaning of the term. I wonder why?
The Father is not the Son or the Holy Spirit.
The Holy Spirit is not the Son or the Father.
The Son is not the Father or the Holy Spirit.
Are you saying that Mary is the mother of the Father or of the Holy Spirit?