This theory is starting off with the assumption that Beledar really is a shard of the World Soul. While we know not all of the information in the Chronicle is true anymore, it outright says that naaru and world-souls were born from shards of Light thrown throughout the cosmos at the birth of the universe.
Like the naaru, the Beledar has both a Light and a Void form, suggesting this might not be an unusual state for a world-soul to end up in.
This brings me to the Black Blood. The PTR reveals that N’Zoth did not create the Visions of N’Zoth using his own power, but through the power of the Black Blood, suggesting that the Black Blood is not simply from one of the Old Gods. The Black Blood was said to have been dislodged by Sargeras’ sword, just like something else did in the same expansion N’Zoth started that trick - Azerite.
Azerite is known to be the blood of Azeroth, but if world-souls shift between Light and Void, could it be that the golden Azerite has a shadowy counterpart in the Black Blood? Both Azerite and the Black Blood have been used to create incredibly powerful and volatile weapons.
We also know that the Ethereals heard a Radiant Song from their own world shortly before it was destroyed. We’re hearing a Radiant Song from Azeroth as the Black Blood is spreading and the Beledar is beginning to shift into the Void.
Maybe the Shadowguard and the Order of Night aren’t so much traitors to their worlds, as much as those wrapped up into following its darker half. Perhaps K’aresh wasn’t so much “destroyed” as its world-soul was birthed during its Void phase.
This is clearly still not a good thing. Even if it’s a natural event for world-souls, it still clearly would not be a healthy one and something we would want to avoid for Azeroth. The prophecy of Renilash, then, may not be so much about a cosmic showdown between Light and Shadow, but Azeroth’s final decision as to whether it will be born in Light or Shadow.