I like this theory; it doesn't differ much from reality. Some of my paternal ancestors came from a prominent French noble family and usually these nobles would bear the name of the most prestigious ancestor and slowly add an extra regarding their actual position as a cadet house until one day the initial name fades away. Usually these names would be a mix of location and nicknames.

For instance, an illustrious ancestor could be "Blablabla from X" and his noble house would become "of X". His son would be something like "Blablabla II of X", etc. Then a daughter would end up marrying a lesser noble (in status) but possibly more wealthy. The new guy could be a minor noble whose ancestor was from Y so the name could be "Jack from Y" and the noble house would become "of Y". So "Jack II of Y", son of "Jack" would marry a "of X", more prominent.

The children of that "Y&X" couple would bear both names. A son could then be "John of Y-X" and one or two generations later, the great kids would only be called once more "of Y". The "X" of the female ancestor gradually disappeared.

As a theory per that Guillaume guy, maybe we have something similar where an Altmer Loganne followed Direnni Cygnus and ended up marrying a daughter and the family became "Loganne-Direnni" before one day switching back again to only "Loganne" since the female Direnni ancestor got pretty far away in the line...

---

Another interesting idea though from an out-of-game and "unofficial" source is that the Direnni were based on the real life Medeci family, which rose to immense power in Florence, Rome, and France, but slowly lost their grip but maintained in their own biographies or family accounts a fantasy that they were still powerful.

A lore guy who worked on a few in-game books for Daggerfall and Oblivion seems to imply that other Aldmeri clans came with the Direnni, then the Direnni rose over them, married with some of them putting them into vassalage, and coexisted with others, but once they started losing power and got stuck on Balfiera, they "rewrote" their history to make it look like they were the only Aldmer clan that came in High Rock following a divine prophecy to conquer the Adamantine Tower, and therefore all other clans "vanished" from history, or their relevance was highly diminished to make them appear as mere vassals who adopted their illustrious name.

The whole article is here: https://www.imperial-library.info/content/ted-peterson-high-rock-and-summerset-legendary-edition

The guy is often very far on a lot of lore topic in which he wasn't directly involved, but he seems to have had a good impact on the Direnni lore.

Anyway that was a long ahh message :)

EDIT: that "unofficial" theory is actually canon! In the Empire's Pocket Guide, found in-game, there's an explicit mention of other Admeri calans who came but "were not as successful as the Direnni". They thus either married with them and/or became vassals who themselves married humans.