I mean, I've seen a number of arguments as people cast their votes that that could be effectively rephrased as: "Belegar has been with us through thick and thin, and we owe him."
I've seen a lot less of that when it comes to disclosing to the Colleges.
There's several factors at play, I feel. First, we did in fact do this in Belegar's home and unlike the Colleges his people are not directly dedicated to studying magic. If we'd done this experiment in Altdorf, we probably would tell the Colleges with no reservation because it's the responsible thing to do.
(Though of course, we wouldn't be making hardcore magical experiments in Altdorf unless we were more comfortable there.)
I'm gonna second Mopman on how Belegar's an individual we know very well compared to the generalized Colleges. It's easier to put your trust in a single person than in an organization. We
have interacted with dozens of Magisters and Lady Magisters on many occasions, but we don't actually know that much about our fellow Greys on an
individual level, which still works out because Greys are
institutionally trained to handle paranoia.
If we'd directly worked under Algard the way we have under Belegar, maybe people would have less reservations. Who knows.
So how did we manage to buy Esoteric Ranald books?
Because we're a Ranaldite with connections with the Cult of Ranald (via Heideck). It's that straightforward.
Mathilde has Ranaldan and Ulrican contacts that let her get up to Esoteric with them, but that's it.
Wait, it's been confirmed the people of Ulrikadrin can help get us to Esoteric with Ulric?
And those are only for Imperial books, right?
I don't think Mathilde's Ranaldian connections extend to Bretonnia, and I don't think Ulric is worshiped at all there. And Dwarves don't have anything on human deities other than Sigmar, I think was what Boney said. So yes, presumably only for Imperial books.
...As far as most human religion topics go, we are in the uncomfortable position of needing to invest actions for full access to topics that won't typically come up that often.