Faolyn
(she/her)
In real world mythology, especially in Goetic demonology, lots of demons are the demons of something, but often because they're credited of having taught humans how to do that something (which is actually really funny to read at times, because you have demons who taught humans how to make cosmetics--demon prince of lipstick, I guess).Personally, if I did imagine redundancy between demons and gods, I wouldn't get rid of either one. I'd simply make demon lords and archdevils more generic. Big bad rulers of their planar layers, not the Archdevil of Tyranny or the Demon Lord of the Undead. Sure Asmodeus might be tyrannical, but only because he's LE. Orcus might love undead, but just because he's into them. Only the gods would rule over aspects of the multiverse
So I see no problem with Orcus being the "demon lord of the undead" because he's the one who initially developed the animate/create undead and similar spells (gods don't have to cast spells to do things). But as you say, he wouldn't actually rule over the aspect of the multiverse that is undeath or undead. It's a perfectly fine distinction between the arch-things and gods.
And it would work well to explain warlocks versus clerics, because the arch-things are actually just giving the warlocks copies of the spells that they made, instead of gods who are actually imbuing magical power into their clerics.