If Vivec and Talos were always gods, then couldn't that mean by mantling, you are merely taking the place that was rightfully yours all along? Tiber Septim didn't just mantel Lorkhan, Lorkhan was Tiber from the very beginning and made Nirn so he could be?

Well lorkhan came up with the idea of mundus so that all mortals could acheive "divinity" through CHIM

Possibly. We know that when Vivec ascended, he changed history so that he had always been a god. However, this seemed to be a result of the use of the Heart, a fairly unique method of reaching divinity. There are certain details in Tiber's mythology that would support his retroactive divinity (both Pelinal and Reman mention Talos, way before his actual ascension), but Tiber is a complicated case, since he first reached CHIM and afterwards became part of a Lorkhan-inspired composite god (at least, that's how I understand it). I guess if your divine origin is Akatosh this is a given.

Well, in Tiber's case especially. He was the snake's dead skin before he took Lorkhan's spot and power.

On Vivec's side of the equation, remember that he either considers himself the Anticipation of Mephala, or he considers Mephala to be his Anticipation (I honestly keep forgetting which way this is supposed to go). As far as the Temple's doctrine is/was concerned, Vivec is the godly coming of Mephala, or the god that Mephala was supposed to be, or something. And Mephala, as a Daedric Prince, has existed longer than Creation.

That said, while Mephala and Vivec are considered aspects of one another in many ways, they are also acknowledged as different beings--but on the other hand, so are Talos and Lorkhan. So... does that help?

I personally like to think that the Anticipation theology is Temple propaganda with no real substance, meant to ease the Daedra worshipers into the Temple doctrines.

Isn't Talos unique to the current Kalpa though?