Well, we know Shenhua is planning for a big conflict, so it's not a outsider problem. We also know that the sects are also having a role as a government which gives them a degree of autonomy.
However, the big thing is that Shenhua is alone. She might be, personally, secure, but the minute she is gone, Renxiang would have to be strong enough to hold the reins. She has no one to help her but the allies she herself make, and she has tens of very, very pissed violet/prisms who want her head.
She might have a couple hundred years to solidify her claim, but right now it's unstable as hell, and the Cai clan is as much as gone unless big war/etc happens. Or unless Renxiang shows herself as much of a monster as her mother one way or another.
Huh, we know how it usually works: we go around and ask an indigo/violet to take us on. If we ask the Bai, unless we are a direct vassal of the heir/head, we'll then have to ask a specific Bai.
That's how all the vassals works. We get a piece of land they own, etc. The difference with Cai is that she would give us more resources so that our house would not be fodder.