A. All he knows for a fact there is that we're messing with grief. Suggestive but not outright proof. Let's not give him proof.
Messing with grief as cover for knowing everything...I'll address what he knows in C.
First, asking papers/materials on grief goes excellently with that cover.
As of now, it is enough to either make him think we want more OR that we already know what we're doing with grief, depending on results - in particular, the latter towards Hildegard. (If he didn't know initially, then he sure as hell has noticed by now, after it visiting several Meguca gatherings and us replicating the effect.)
B. That we know everything. Absolutely everything.
Megalomania *whack* We do not know everything, or even better than anyone!
We are interesting anomaly due what we know, granted. If being an anomaly was sufficient for active targeting we'd have been kidnapped already (amongst others).
As for the secrets we know...
Lot of it isn't really useful to him beyond "Sabrina knows X" way, and not really relevant - why would we reveal some more meta secrets.
Things such as "not real" don't have particular impact on his position, after all.
The worst secret we could reveal, imho, would be Homura being time traveler - expect it is implied that Kyubey already knows "she is making things worse" and does nothing.
Other secrets aren't particularly harmful. Us wanting to make a better MG system? That is not even a secret at this point with our actions in Sendai, for instance.
C. Honestly? There's a lot of leeway and it could be anything from "not sure what she's playing at" to "She knows about the Witchbomb, and more- I want to find out what that is"- but I'd rather not flat out confirm exactly what we know, or give him knowledge that he could use to tear down, say, Homura.
He also knows that:
A: We are a do-gooder, out to save puella especially and humans in general.
B: That we made wish to control grief, specificially mentioning puella magi and witches in addition to ourselves.
C: That we experiment with grief seeds and grief both.
D: That we are manipulative and preternaturally knowing.
E: That our existence and potential are both anomalies.
F: That we're willing to give him grief.
At the very least, it makes it seem as if we'd balk at nearly nothing to save puella magi from eternal toil versus witches - including not only good things.
He certainly at the very least suspects that we know what we're doing with Hildegard, given us repeating it, while willing to be cooperative.
Tearing down Homura...You mean with something like "Sabrina is willing to let Madoka make a wish"?
Tearing down with witchbomb is something he can do at any time.
D.Because every single time Kyubey's known something about someone's motivations, that's ended horribly? The less he knows about us, the better.
He already knows our main motivation. We've both made a wish to him and used him as relay to convey it.