Hive resurrection is a lot more common than Ana Bray would have you believe. Whether it's hiding one's death, having your soul rerouted to your Throne, undoing one's death personally, bringing others back through your might/magic, or making a pact with Xol, the Hive have been comming back from beyond the Veil for eons. They thrive on death and undeath, Ana just doesn't know anything about translating Hive apocrypha or understanding their social implications. Xol's resurrection isn't so much forbidden as it is simply tasteless in it's implementation, it takes no might or skill if your power is sourced from someone other than yourself. Even though betrayal of the Sword Logic is heretical to the Hive, Knights do so whenever they summon their shield. Oryx didn't crack down on this (unless he took one) because nothing is sacred to the Hive, not even their Sword Logic. Oryx only punished Nokris because he wasn't going to have that kind of disrespect in his family. Nokris was already banished even before he conducted the resurrection ritual, meaning Oryx was probobly testing his resolve and tenacity by sending him out into the world on his own, like he did with Crota. What Nokris did would be akin to if your teacher gave you a test and instead of you taking it, you spit on the paper, threw it back in your teachers face and flipped your desk as you left the classroom. You could see why Oryx would be upset with him.

I replayed this as well and realised that Xol is most likely still kicking around, and I know he’s in the whisper, but I think he has a bigger role to play

It actually doesn't defy anything. Sword Logic is basically "I killed you because I'm stronger, and therefore have more right to live." The Hive regularly use resurrection, and use the knowledge they learned from their death to become better.

I don't really keep up with lore, but there is a scannable on Titan and your ghost mentions that there is something big below the water. Maybe it's Xol or something similar?

Illuminati confirmed