I've been thinking about how a mirror heresy would work (not familiar with the Dornian Heresy), but I always find it difficult to get Magnus on the right side, he does everything wrong. What if instead of Horus, Chaos would have turned any of the other Primarchs. The easiest to explain would be the Lion, since Caliban is already swamped in warp-fuckery so it's less of a stretch that he got a bug there. Istvaan would be effectely be reversed, with Iron Hands, Salamanders and Raven Guard stabbing the poor Night Lords, Iron Warriors and Word Bearers in the back. A lot of it would be mirrored, e.g. the Night Lords swapping places with the Raven Guard, Wolves and Thousand Sons still tearing itself apart but with loyalties swapped.
The "Dark (Angels) Heresy"
Traitors:
Lion El'Johson (chaos undivided, killed by the Emperor), Sanguinius (Demon Prince of Khorne), Ferrus Manus (Giger-esque Demon Prince of Slaneesh), Rouboute Guilliman (Over-ambitious Demon Prince of Tzeentch), Vulkan (regenerating Demon Prince of Nurgle), Jagathai Khan, Leman Russ, Rogal Dorn (Demon Prince of Chaos Undivided), Corvus Corax (killed by Imperial Assassin)
Loyalists:
Horus (dies saving the Emperor), Lorgar (killed on Istvaan, in 40k saint second only to the Emperor), Perturabo (decimated on Istvaan), Konrad Curze (nearly killed on Istvaan, goes full Batman after most his legion gets slaughtered), Angron, Fulgrim, Mortarion, Magnus, Alpharius (always loyal
killed by Dorn)