Mostly because the clans are in the general habit of secrets, and the actual texts either revealed or hinted at deeper clan secrets, like techniques.
Kaguya is, apparently, a complicated person, who used to want to bring peace to the world but decided that the world can't have peace as long as humans are on it. Rian is descended from her, and at least somewhat resembles her son and grandson, which is presumably part of why Kaguya pays more attention to her than to others.
They don't know anything at all, except that Kaguya wasn't alone at first. They actually have no clue whether chakra was present before the God Tree, or exactly how it spread post-Kaguya, or why she has chakra, too.
Kaguya's history isn't broadly known since most people simply aren't interested, and people are unlikely to associate one with the other, though it's possible someone would make that leap. Her world and magic sounds dramatically different from Kaguya's, though.
The Sage is one of Kaguya's sons, yes. The Ten Tails' chakra sort of - contained an imprint of it, so the Sage essentially sealed its mind and soul into himself? On his death he split it into nine parts, which formed the bijuu.
Yeah, someone - they're not entirely positive who - gathered the nine bijuu and combined them back into the Ten Tails and somehow used that to unseal Kaguya. Kaguya turned the reformed Ten Tails into the God Tree when she cast her illusion over the world, it's regathering chakra and distributing it to her they think.
Kaguya can grow her bones at incredible speeds and use them as weapons. Hers turn anyone they hit to ash, the Kaguya clan's didn't do that.