Here is something that became clear to me recently:

So Jolethe, in 3E 145, as sister of Pelagius III and daughter of Magnus Septim, inherits the throne of Solitude from her elder brother when he is crowned Emperor http://www.imperial-library.info/content/madness-pelagius.

She and her lineage then drop off the face of Nirn. Or does she?

In http://www.imperial-library.info/content/brief-history-empire, Cephorus II is described as being elected to the Elder Council in 3E 247 after the disinheritance of Andorak Septim (to recall: Andorak was the son of dunmer emperor Uriel IV, who was the child of Empress Katariah and her Breton consort Gallivere Lariat). As opposed to Andorak, who had no blood relationship with the Septims, Cephorus II is described as "a cousin more closely related to the original Septim line."

So what? Well, who exactly Cephorus II was is described further elsewhere: in http://www.imperial-library.info/content/fall-usurper, to be exact. Here, in the context of talking about Breton apathy to opposing Haymon Camoran, Cephorus II is described as a Nord with "Skyrim and Morrowind sympathies."

Given what we know about his origins, it seems likely that Cephorus II would have had a direct familial connection to Jolethe. This would fulfill his requirements of being both a cousin to Andorak as well as being more closely related to the Septim line, being a direct descendant of Magnus Septim.

Now the wording of A Brief History also implies that Cephorus II was not a Septim. This would further fit this theory, as Jolethe would have likely taken the name of her husband should she have married.

This now leaves me with two important questions:

1. What happened to the descendants of Jolethe in Solitude? Given that they would have lost the Septim name, it makes them difficult to track. Might it just be possible that later identified rulers of Solitude, such as King Thian (of Pocket Guide fame) and King Torygg might have counted Jolethe as a direct ancestor?

2. If Cephorus II was not a Septim by birth, how did he (or his progeny) acquire the name? Uriel IV was adopted into the Septim family by his half-brother Cassynder Septim. Besides Andorak and his progeny, there wouldn't have been any Septims around to adopt Cephorus II into the family so that he could claim the surname Septim. Did the Shornhelm Septims have to be appealed to? I can't imagine, at least during Andorak's lifetime, that they would ever secede to such a request. Did the Elder Council just 'declare him' a Septim? Sounds pretty dubious.

Of course, Jolethe and her descendants may have preserved her maiden name through the generations for the purpose of political clout; but then where did the Septims of Solitude go?