[Phoebus]

The Inner Circle is a very small group of people. It includes Azrael and the Successor Grand Masters, for certain. Someone with his hands on the 4E Codex can look to the page that outlines the Chapter's structure; it qualifies what Dark Angels are in the Inner Circle besides Azrael (I believe it's just Ezekiel and the Interrogator-Chaplains).

Members of the Deathwing are inducted to some of the secrets of the Chapter's history. Librarians, (non-Interrogator) Chaplains and Company Masters are privy to even more secrets. They do not, however, possess the knowledge of the actual Inner Circle.

As for keeping the secret? I don't see how this would be a big deal. The Imperium had undergone the Heresy, and was still undergoing the Scouring. They had a million worlds to keep track of--a great number of which were still burning, a wrecked infrastructure, ruined lines of communication, etc. How would anyone have even blinked if, at that point, the Dark Angels reported that a single planet, just one of maybe thousands that suffered similar fates, had been destroyed?

 

[Phoebus]

Oh, I hear you. I guess I look at it this way.

1. Heresy ends. The Imperium is in chaos (no pun intended), shambles, etc.
2. The Dark Angels, led by the Lion, head to Caliban.
3. Luther's treachery is revealed; Caliban is destroyed.
4. A certain period of time passes, by which time the Dark Angels arrive at a consensus for their secret.
5. That period of time falls during the Scouring. During this same time, Vulkan's fate remains unknown, Dorn and Guilliman are (presumably) killed, and, more importantly, Corax, Leman Russ, and the Khan depart on more or less unverified journeys confirmed only by their Chapters.
6. The Dark Angels "reveal" to the Imperium what happened to the Imperium probably some time after knowledge of one or more Primarchs departing became common to other Astartes.

How plausible would all this be? Enough that the Dark Angels got away with it, I guess.

Ultimately, I think it was a matter of there being enough insanity going on at the time that a planet (belonging to the Imperium for only fifty years) being destroyed and a Primarch disappearing simply weren't considered that wild an occurrence.

Just my two cents' worth, though!