If you read Vehk's partial interview and THEN read the Five Songs (especially the part about "Vehk the devil" blasting Wulfharth) then you should be, in the very least, a little less confident in your idea.
Because if you read the partial interview (the part about Vehk killing Tiber Septim) and the part of the Five Songs (the part where Vehk blasts Wulfharth to ash), then you'll learn Tiber Septim was at Red Mountain. Because if you read these two parts, you'll learn Tiber Septim was at Red Mountain. Because if you read what I've stated earlier, you'd see that Tiber Septim was a "role"/oversoul in a sense. "Heresy" fleshes out this idea in a clandestine manner. Behold:
"Before Cuhlecain can be crowned, Hjalti secretly murders him." (The assassination story is embroidered -- now it is popularly Talos' own throat that was cut.)
Do http://www.imperial-library.info/mwbooks/lessons.shtml#11 to read why the holy institution of marriage, that is, the union of two into one, is synonymous with the unholy transgression of assassination. Heresy notes the confusion of Tiber Septim's identity again near the end. Once again, behold:
"Still, there are conflicting reports of what really happened, and this is why there is such confusion over such questions as: Why does Alcaire claim to be the birthplace of Talos, while other sources say he came from Atmora? Why does Tiber Septim seem to be a different person after his first roaring conquests? Why does Tiber Septim betray his battlemage? Is the Mantella the heart of the battlemage or is it the heart of Tiber Septim?"
"How was Tiber at Red Mountain," you ask? Because Tiber ain't a real person. It's a role. Wulfharth played (a part of) that role.
edit: Cause I'm such a generous gent, I'll do some more quoting. Concerning assassination, behold:
"...marry it and by that I mean secretly have it murdered."
http://www.imperial-library.info/obscure_text/chaos.shtml, behold:
"Assassination, [the Morag Tong] say, is the purest celebration of joy or living."
During a marriage ceremony, what is commonly said by the preacher?
"The two become one flesh..."
Honestly, TRULY am not trying to be a pompous ass. I TRULY want you to, in the least, question your current idea and attempt to contemplate the one I've submitted to you. Because when you think about it hard enough, it makes sense.