Do we know it's incarnation and not mantling? Yes, we're told they are different things, but I'm thinking about what we actually experience in the game. There is definitely an active element to becoming Nerevarine - the failures attest to that, and also the role of the Sermons as instructions to the player on how to be successful - becoming Nerevarine is a teachable thing, to some extent.
What would mantling Nerevar involve? Can you mantle someone who was not a god, or does that make no sense? Do any of the player's actions echo those of Nerevar, or at least, actions that occur before the Cavern of the Incarnate, which would, I guess, have to be the culmination of the process?
Not saying I definitely think it IS mantling, just considering the possibility.