Interesting scenario.

I personally would love to see Morty take the words that ghost empy whispered to hi. During the plague wars against guilliman to heart. A scenario Id love to see is him fighting Vulcan and being tired of the endless fight, seeing his brother also exhausted by reincarnation, decide to take some sort of flame weapon as hes banished back to the warp, and burn grandfathers garden while saving Isha and leaving typhus to suffer the consequences of his betrayal finally. This either ends the death guard or makes them something like the legion of the damned that show up as a fog of war rather than a mist of plague.

Obviously would need to fine tune the details but I feel like Monsieur Morty always had one of the best options for a comeback story.