Too servile to be Ligier, I think... That would be fun, though.
He's had ages to get worn away, and lost his memories to boot, so it's hard to tell. The exaltation sheltered him, but only so much because he was a hanger on and not part of it intrinsically.
Also worth considering that the nature of Molly's relationship with him is very different than the one he'd have had with most viewpoint characters.
He's basically part of Molly's soul structure now. Molly is the fetitch soul of the primordial she's becoming, so the relationship is more like how one of Malfeas' third circles would interact with Ligier than how Ligier might interact with someone external.
Not saying he definitely is, but if this line of reasoning has any substance to it then Usum would have to be something Malfeas considered worth special preservation.
This particular tinfoil hat is one I'm reluctant to take off, because it makes a lot of sense to me as a backup plan to the backup plan for the Yozi.
Primordials are identities so large they exist beyond the scope of a single soul. They're one entity and many at the same time, but the "I" of their "I think therefore I am" is the oversoul.
From that perspective becoming part of a new primordial growing out of an infernal could be seen as a form of change more than a form of subsumption.