I'm having a hard time unearthing any information on what religion the Line of Vol or the Mabar-aligned factions in ancient Aerenal would have followed. The two leads I have are that the Qabalrin reportedly worshipped the Shadow (according to Giant texts), and that the Bloodsail elves have a similar structure to the religion of the Deathless.
So the actual fact is that I intended the idea that the Qabalrin WORSHIPPED THE SHADOW to be inaccurate. Here's the relevant quote, with some highlighting...
The Qabalrin were SAID TO BE disciples of the Shadow, one of the evil deities of the Dark Six. They possessed arcane knowledge rivaling that of the giants, but this magic was SAID TO BE dark and terrible. LEGEND SAYS that the Qabalrin were the first wizards to practice the art of necromancy, and that the elves of the line of Vol based their work on Qabalrin tradition. ACCORDING TO THE TALES, the gods destroyed the Qabalrin as punishment for their blasphemous practices and their allegiance to the Shadow.
In my opinion, this is propaganda and myth handed down by the Cul'sir, who DID worship the Sovereigns. It was easy to cast the Qabalrin in this light, because they did pursue dangerous magics that often came with a terrible cost, and that's the Shadow's domain; they also literally worked with shadow, manipulating the forces of Mabar. But in my view, the Qabalrin were a SCIENTIFIC society—deeply rational and devoted to breaking the world on the anvil of arcane science. The Bloodsail Principalities are supposed to be closer to the culture of the Line of Vol, who in turn were inheriting traditions from the Qabalrin, and what's said of the Bloodsail clerics is that they "They shape their divine spells from the raw energy of Mabar". So the idea is that they DIDN'T worship any sort of deity; they believed in arcane science and their own ability to bend the planes and reality to their will through science and discipline. They sought to conquer death—just like the Undying Court and Tairnadal—but largely viewed in an abstract, scientific way.
This in turn reflects the evolution of the Blood of Vol, which developed when elf exiles brought those traditions to a human population that worshipped the Sovereigns and were suffering bitter oppression and harsh living. Mixing chocolate and peanut butter, they created the idea of the Sovereigns as tyrants who inflicted death and suffering on mortals to prevent them from unlocking the Divinity Within. As noted in Dragon 410, the Bloodsails hold to the tradition abstracted faith of the Line of Vol, but respect the Seeker priests because of their roots; essentially, they see the Seeker priests as having a foolish layer of superstition with this whole "cruel gods" story, but appreciate that they are both pursuing immortality.
Source: Keith Discord Q&A