Im going to start by saying answers to this discussion should take gameplay mechanics out of consideration.

It seems to be the consensus that all men descend from either the atmorans or the yokudans. Is there much lore behind how the descendant races came to be? I'm talking about colovians, nibenese, redguards and Bretons. What specific events caused them to develop into their specific race?

To get more complicated, would reachmen be considered more nord than eastern high-rock Bretons?

EDIT: Moved from phone to computer, so reformatted the ugliness.

By the time of the 4th era, perhaps, but I believe the Nords expanded pretty far into High Rock as is, to the point where Nordic culture is more prominent in some locations. The relationship between the Reachmen and Nords is frosty at best, and would try to steer clear from the other more often then not.

Fairly sure the Nibenese are closer to the indigenous men. I assumed the Colovians were basically Nords with a mix of Nedic blood.

I could be wrong but my understanding is that there were local men already on the continent. The forsworn for example or the Cyrodillic races (N and C) were slaves to the Heartland High Elves since time immemorial. All men most likely are not from Yokuda or Atmora because there were rumors of a race of men on Akavir who feel victim to the Tsaesci.

Come to think of it weren't there tribes of men in the Black Marsh who were wiped out by that Thrassian flu plague or whatnot?

Silver skinned men.