The Custodes being shadow rulers of the entire Imperium whilst the Emperor is nothing but a mostly dead, psyker becon is a rather good idea actually.
I think I can fit that into my head canon rather nicely.
What I meant to say was that Sisters have authority to police the High Lords. Vandire is not the last time they may send High Lord heads rolling.
But yes, Sisters can hold a place on the High Lords and I'm hoping that GW actually do just that with what ever possible update that may come in the distant future.
Go back to the ashamedasylum
Well, to my estimate, it's slowly degrading. This means that a 'sudden fall' will not be as severe, in absolute terms.