Well, we still don't know exactly what defeating the Darkin gives. Sure, it gives the host all of the Darkin's power, but in what form? Kayn becomes a Shadow Assassin, but it's because he fuses the power of Rhaast with the power of the Shadows that he owns.
But Varus? In his current lore, he was just an archer. The best archer in Runeterra, but still just an archer. That's why I think, if the theory is right, he could just have taken some Darkin features, since he didn't have a base power to enhance. Now, after the retcon Varus's appearance could be completely different, so it's all fluff.
On the other hand, the necklace theory is actually a good, since it looks like an important item for him. Of course, it could just be a plain memento from his home, but still.
But anyway, until a VU happens, there's no reason to indulge too much. There's so little now, old lore didn't include the Darkin twist (as far as we know), so we pretty much have nothing about him.