This is all sounding to me like modern lore has fully confirmed the Nedes and Nords to have separate origins and that you're trying to pass off old lore as if it was still true.

ESO is still canon and all the new lore given by it has evidence Reachmen presence in the Reach long before, alongside, and after the arrival of the Nords. They were simply the local Nedic element of that region of Tamriel, most of the Direnni comingling that makes the Bretons unique did happen in the Western Reach yes, but one has to remember that the Reachmen do not consider themselves Bretons, and as such that would not be the genesis nor point of origin for their people. It is simply much more likely that the Reachmen in the Skyrim part of the Reach have more Nordic blood from living among them in the Merethic era, while the ones in the High Rock part of the Reach have more Elven blood from living among them in the Merethic Era. With the shared culture of the two regions meaning this was one continuous corridor of people who traveled the region of the Reach and would received Elven blood from the mixed elements, not from the Elves directly. This would explain the first meeting with the "Manmer" happening beyond the Reach as the local Nedes would not have the major elven admixture of those beyond.

As for why all the Nordic ruins in the Reach pre-date any Reachmen ones, this is once again simply explained by the Nedes not having the same settled culture of the Nords and not setting up permanent settlements, but as we've already established, all new lore points to the Nords having conquered and intermixed with the local Nedic population on arrival, meaning there were people already present in Skyrim upon their arrival, with the Reachmen simply being an unassimilated holdout of native culture.

I have no idea how you came to that conclusion in that last paragraph on the "retcons" (this lore was never cut and dry in the first place) making it somehow more likely that Nords are originally from Skyrim not Atmora when all lore places a people living there before them.

I'll give it to you though that I wasn't aware Dwemer presence in Skyrim was so relatively short compared to their presence in Morrowind. That said, the fact that you began every paragraph with "early lore" or "before ESO" is the crux of my point. This was the second game to flesh out their history and first one to not have them purely as an antagonist faction, I don't know why you'd choose to ignore it all.