Wow! That looks as if it took a bunch of research.
In any case, sorry, I remain unconvinced. I say that "come" is not a preposition. You could say that it ACTS like one; but when you look into its past, I think future subjunctive is more likely. Now, how to explain how "(Should) January come" could turn into "come January" is an additional problem for someone arguing MY case. But I think simply to consider "come" as a preposition is a cop out. There are not any comparable precedents. "To OFF someone" or "to DOWN a plane" are instances of prepositions that can cross over into verbs. But "to come" for me is a verb and a verb only. (Well, I guess it can be a noun or act like an adjective). And, of course, ANY word can become an interjection. WERD!
Interested in Language