if they were mating between each other from the moment they became half Mer then they are still half Mer - if they dilluted with blood of even lesser races then yes, they would stop being half Mer
bust most probably majority kept themselves to the rest of breton simpletons
Actually, the evolution of Bretons since their intermingling with the Direnni Altmer is well-documented in the lore.
After driving the Direnni out of High Rock, Bretons mingled with several populations of human that sprung up in places such as Colovia, Skyrim, the Niben Bay region, and later Hammerfell. Later human-dominated empires ensured that Breton bloodlines mixed often with those of other humans, as opposed to the more geographically (and culturally) distant elves.
The Bretons aren't like the Altmer in terms of keeping their lineage "pure"; they were quite happy to settle down with any sort of fellow human if the mood took them. Thousands of years of this all but guarantees that a true human-Direnni half-breed no longer exists at the time of 4E, and likely long before.
As for the dev Q&A, I'm disappointed to see that they'll basically be glossing over Redguard class struggles. They've stated that Morrowind's great houses will have quests related to them; why not a quest or two focusing on the friction between Crowns and Forbears?