Is he a fanatic? His VO is about finishing what he started, which should be a reference his dead daughter and the Dark Justiciar War, but we also know that Ketheric tried to get the Sharrans at Grymforge to worship him over Shar, a blasphemy. As far as I know, nothing from the Moonrise curse has anything to do with Mind-Flayers or the Absolute, which appears to be fairly recent. Not that they aren't connected.
Also like you said, if Shadowheart is working for the Sharran's then Shar and the Absolute are at odds.
My incomplete understanding of the lore is problematic here. But, netherese magic is shadow magic, which may or may not come from the shadow weave?
The tadpoles are produced using this magic. Deciding factor here if that means Shar has to be involved...
Ketheric is a clear oath breaker, then. I suppose, he wasn't about to take up another oath? Can someone just forge another? Still, he seems so much like an incredibly vengeful guy. Maybe that's compatible with being an oath breaker. Revenge to honour something vs revenge /in/ destroying something out of hate?
I know at least that Paladins need to make their oath to/before some power. The absolute may well sponsor him, or just have similar interests. It would make him a more interesting villain to not be a mind controlled quasi slave.
The thing with Shadowheart is, I don't know if they took her memory because she wasn't on board with the tadpole insertion. So, the journey to Bg is a lie and they set her up to be abducted. Especially, if the artefact had bonded to her (planning its break out) and made her reject the absolute's influence. Or maybe, the tadpole was just a safety measure in this express delivery of the artefact. If their faith is being puppeteered by absolute spies, it could be trouble.
Possibly, the absolute just has a netherese bastion. Shar does oppose them. But, the missing memory is so fishy. Something is going on here. Past cleric of Selune is not someone I'd have carry the artefact, unless the artefact was captured by the Selunites and its attachment made this the only way to transport it. Something like that.
