As you can find "protestants" who will believe most anything. Lumping a few fringe groups that don't believe in the Trinity in with "Protestants" is as offensive as lumping the Mary is God group in with Catholics - except that the Mary is God group actually claims to be Catholics. Jehovah's Witnesses, Mormons, etc., as far as I know do not claim to be Protestant.
Yes, and I would not do that; maybe someone else did. I may have mentioned two things in this regard:
- That the Trinitarian theology itself is a product of the Church, which was formed in no small measure in discussions about Mother of God and such. It is not a self-evidently scriptural belief. The fact that there are non-trinitarian groups that profess Sola Scriptura is witness to that, even though they should not be lumped up with the Protestants.
- That there is enough fracture inside legitimate Protestantism, even though all subscribe to the four Solas. Compare, for example, Lutherans and non-credal baptists. While these differences mught seem small to you, they indicate that at least on these matters, the putative leadership of the Holy Ghost of all believers is not there.