Sebellianism does not enter into it. Your use of Dunsmen is a red herring. We’re talking about Theotokos, and not the immaculate conception.
There are plenty of resources and studies out there (even on the internet) of why the Catholics see this as entirely plausible and in fact biblical.
Nothing personal...I do appreciate your comments. I enjoy the intellectual stimulation...and to be provoked to study God’s Word more! It is the only thing that excites me in this wicked Generation.