by asserting that Orpheus did it later (on what grounds??)

I'd have thought that Orpheus' descent-and-return would have to precede Odysseus' necromancy—I'm not even sure I would qualify it as a descent to Hades, because he is sailing out of the mortal world, past the White Rock and the Gates of the Sun, but he's also calling up the dead: not going down to meet them—because didn't Orpheus sail on the Argo, along with Achilles' father Peleus? That's long before the Trojan War, never mind the heroes' homecomings. I have no idea where Theseus fits in temporally, however.