"Sul" as a title seems to work until you consider Sul of The Infernal City, whose name is simply that.
Linguist point of view: I too have always thought that those with Sul in their name wear it like a title, like countless Serjos for example. It's possible that at some stage, this title just dropped / was forgotten. This would've been followed a period without any Suls. Later, accidentally or according to heritage or whatever, the word might've gotten a new use as a regular name.
Or, the title never dropped, just that its original meaning was forgotten and mer began to comprehend it as a regular name. Language can change like that due time.
Or, every Dunmer knows it used to be - and still might be - a title, but it became a name nonetheless. Like, for example, that aforementioned Smith.
Or King.
Just speculation. Just to say that it's not necessarily wrong to assume it is (or was) a title (as well as a name).
As for whether there's a Big Mystery behind the name... I don't think so. The "many famous Suls all around lore must mean something" argument kind of flukes, unfortunately, because
1) it might be a significant Dunmer family. Or many families with the same name. Heck, it might even be, like, avatars of a same God or something, just that this is unlikely since there seems to be nothing to back this up. Good find, although we don't believe there's anything to it. Just look up for new evidence for their connection and present it here again if you find it!
That'd teach us damn sceptics...
2) lore doesn't tend to list anything but big events and famous names anyway.