[X] Write-in: when speaking to Magnus, "Look, I understand why you are terrified of revealing this to Mortarion, I am tempted to let you wipe my memory for it...but remember how you felt when you confirmed about the Archdaemons from me and how much it hurt that I kept it from you? Imagine how it will gut Mortarion when he finds out you kept this from him just after you and he were starting mend bridges, as I can only imagine that happened when he mentioned going to you for aid.
[x] Write-in: Inform Magnus that we are telling Mortarion and Horus, yes WE as in the both of us. Understanding that he is incredibly worried about doing so, we nevertheless must inform him, and do so now before this gets out of hand, which it certainly will, as there is no possible way the Plotter won't use this against all of us if we leave here like this. We'll inform them together and then work with Horus to temper Mortarion's initial reaction. Something must be done, but that something must be viable. When Mortarion inevitably asks as to why Magnus didn't share immediately, be truthful, Magnus was terrified of the results of such, and so Kesar who he trusts to express his fears and work through his thoughts before attempting to explain things to a more difficult audience in Mort and Horus.
[X] Try and make Magnus share what he found. Trying to get Magnus to share the information himself won't be easy. But if Mortarion hears it from him, then perhaps their relationship may be salvaged. However, there is a large chance Magnus may not even decide to share this information even if pressed.