I'd completely missed they changed succubi to default Neutral Evil this time. Not that it matters for the overarching point.

As for "most devils" conspiring against their superiors, that's part of what I mean when I say newer editions are bad at explaining the difference between Chaos and Law. Devils are, supposedly, Lawful Evil incarnate - them having no in-group loyalty cheapens that and makes them less distinct from demons and other such creatures. Their malice should be mostly targeted towards those outside their group bias - which is why it matters whether the usurper king is "one of us!" or not.

This loops back to the double-bind described earlier, where the usurper king either unflinchingly pursues their dream, bringing Hell on Earth that way, or they falter in the face of their niece and the demons break from his control because he wasn't devilish enough.