Honestly, if I was in charge of current lore, I’d play an Uno reverse card on the Shadowlands writers’ unreliable narrator excuse, and declare their lore unreliable narration.
In my hypothetical revision, the First Ones only created the Shadowlands, but wanted its denizens to believe they were responsible for the whole cosmos, so they would be venerated by the denizens of the Shadowlands as the supreme gods of all creation in their eternal and unquestionable Purpose. The other Zereths were a lie — there was only Zereth Mortis. Then the First Ones disappeared, leaving their creations to unquestioningly go through the motions.
If I felt particularly spiteful, I would say they were an advanced ancient civilization that created the current (artificial) Shadowlands as a replacement for the original afterlife, which is where mortal souls would naturally go after death if not for their design. The kyrian’s purpose, unknown to them, is to snatch souls and bring them to the artificial Shadowlands before they get the chance to pass on to their proper, natural afterlife.
This wouldn’t undo the damage, but it would contain it to the corner of the setting that most players don’t care about. The more radical version would also present a way to restore the universe’s proper afterlife later; the Shadowlands as we know them would remain a separate plane of existence, allowing the already-dead to continue to exist there in peace if they want to, but would no longer function as an afterlife.