Just one comment to "loyal primarch defecting". Well, we don't have many of them alive, right ? And I can clearly see where this rumour is going.
Certain Lion waking up and seeing Guilliman trying to build new Imperium (again), going all "meh" and taking his over-secretive chapter somewhere else. With Cypher of course !
But making renegades of one of the most favourite chapters ? Not likely.