I've always thought mantling was merging with a god like CoC merged with Sheogorath in SI. But if Tiber Septim mantled Lorkhan how come he became Talos?

Mantling isn't merging, no no no, it's a much more exotic, multiform beast.

In TES, the Mythopoeia is major theme, and an even greater force.

Conventionally, Mythopoeia is but hyperbole: the tendency of reality to be fantastically misconstrued overtime into myth. There's always a grain of truth in every myth, afterall.

Thing is, it also works in reverse in TES, myth can be so fantastically misconstrued that it becomes a reality (or a single version of reality). The lies become true, and single grain becomes a burgeoning field of ripe wheat.

Why this is, we're just not sure. It could be that the Godhead just loves a good yarn. Point is, Mantling writ-large is when an individual wraps him/herself in a gossamer cloak of myths so fine and so thick that nobody, not even an omniscient intelligence can tell the difference. The Mantler walks so perfectly in the habit of their target, that the Mantled can't help but fall into line behind them. Then Convention gets the memo and takes a hike.

As for Tiber becoming Divus Talos and not Shezarr-lite? Well, Talos is actually more than one person. I think that might have something to it with it.

Just because you become something, doesn't mean you stay that thing. Talos hijacked a lot of Lorkhan's power (like Vivec) used that to achieve CHIM, and became his own god.

"Walk like them until they walk like you."

Nowadays, Talos walks like Talos and Lorkhan follows suit.

I'm not quite sure what you mean. We know that the CoC "walked" like Sheogorath until he became Sheogorath and Sheogorath became him. so they fused together, and became one. As far as I understand it. I mean there are not two or more sheo's. And how is Talos more than one person? Still though shouldn't they fuse with Lorkhan, if they mantled him?

Sorry for the late reply BTW.