Honestly, I'm still unsatisfied with Grey Angel. It introduces a "mystery savior" who is obviously implied to be one guy but can't really be because his actions don't vibe with the ones shown elsewhere, all while retconning the timeline hard and rendering Luther as a Horus fan - something that has literally no bearing on his secessionist motivations. If he was a fan of Horus, he'd probably have sided with him, but he didn't.
On top of that, it retcons Luther and co as having been sent back later than originally established (something we had to deal with after that point) but actually knowing the Sons of Horus surprisingly well, to the point of recognizing his captive - who is not a "true son", in terms of facial features.
And lastly, it establishes that Malcador knows of the situation on Caliban, and that he was able to simply send two of his Knights Errant - one of which barely managed to get back to a state of questionable sanity - over to pay them a visit, while the Lion's emissary fleet took another few years to even get there through the turmoil in the galaxy.
There's a lot of good stuff in that audio drama, but I can't help but think of it as a retcon piece, with a lot of superfluous added bits that do nothing but muddle the waters, than something that really needed to exist.