I like the theroy alot, and mostly agree with it as being a very, very good plot twist if ever there comes one. The earring and mother sob story will come back by the end no matter what...
.. But for arguments sake, hows this. It was touched apon in the first book and again in the second book when Vin is talking to TenSoon about how to control Kandra. He say's somthing to the effect that her powers are so strong because of her none diluted blood line, which stemmed from the high prelen, who would have been breed for the position. That with durilum (spelled incorectly of course) made her able to push through the years, to back were allomancy was more pure. Unlike with Keslier who just got very lucky with his heritage, and through a couple of years of practice and dedication manifested his power through force of will and skill, not brute force. Because, while you may say Vin was more powerful, Keslier had the fienesse (Spelled wrong as well) that one could only dream of having.
Again, I think I like your theroy better. But the Idea that Vin has the power because she is been singled out doesn't sit well with me. I like the idea that others could be as strong as her, but are just as rare because of the delution in the blood lines.
ps. Elend will be the strongest of all if my theroy holds true, for the strongest allomanceirs would have been the ones created directly from the source (the Well) and that would make Elend one of those. As I said if my theroy holds true then he will be the most powerful Allomancer to walk The Final Empire.