Question about septs.

[MurrayInGA]

I'm far from an expert but have been going through similar research for 'my' clan, Murray. I think your last point is the critical point: "I've read about clans from companies trying to sell tartan and clan crests, but I was wondering how accurate this information was."
This is by far the biggest obstacle to any and all historical clan study.

Ill cite Wikipedia (usually very poor resource but I think they're correct here): https://en.wikipedia.org/wiki/Sept
"Today, sept lists are used by clan societies to recruit new members. Such lists date back to the 19th century, when clan societies and tartan manufacturers attempted to capitalize on the enthusiasm and interest for all things Scottish. Lists were drawn up that linked as many surnames as possible to a particular clan, regardless of whether there was an actual historical connection to that clan surname. In this way, individuals without a "clan name" could connect to a Scottish clan and thus feel "entitled" to its tartan."

Simply put I'd say that 'septs' are merely lists of families that have had some affiliation with the clan in question(historical evidence rarely cited), implying they "allied"/"swore allegiance" to a proper clan chief and were therefore welcomed as members of the clan at some point. And, just like clan surnames, families sharing that surname existed across Scotland for varieties of reasons. One branch of Cromarty's in Perthshire may have been a sept of the Murray clan while a distant branch of Cromarty's in Inverness-shire were a sept of Clan Urquhart. As there was no singular Cromarty family (chief) their many branches were free to align with whichever clan was regionally advantageous.