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Something else to consider, Horus was Here first.
He's had at least some years, perhaps decades, to gain favorable relations with Kelbor, ensuring the supplies to the Luna Wolves in exchange for whatever his legion could provide.
Horus is, most likely, a better diplomat than we. Our advantage will therefore be reliant that we are a member of the Cult Mechanicus where Horus is not, and crucially the actual divine support of our Machine God. Even supposing we are somehow lucky to pull Kelbor to our side over anyone else's, Horus agents will know.
On the flip side, we should start scattering agents and spies of our own so that we can get out foot in the information door. I believe in the realm of cyber-espionage, information gathering and prying information, perhaps our ability to provide superior technology could give the advantage over Horus chosen spies. At least until Horus finds Alpharius. Especially if we work with Malcador and help him establish the Vanus assassins.
I wonder what it would take to get Alpharius on the loyalist side. . .we obviously will one of the few brothers who has no qualms against trickery and deceit and would consider it a valid tool, in contrast to Russ, Dorn and Guilliman.