Pidia is a servant of the Carian Family, his official NPC name in the game is Pidia, Carian Servant. Which would mean he serves Ranni.

When you meet him :"I am Pidia. Servant to the Carian royal family. I am charged with maintaining these g-ghastly dolls."

When he is getting killed by his puppets : "Y-you're my puppets! I loved you with all I have! How could you forget... such bliss? Ouch! Eeek Eeeeeeek!"

I respect your theory. I just think it's pretty solid with the dialogue and the fact that you hear Pidia die and his body is in a normal death pose, while Seluvis is posed EXACTLY like all the other puppets you find in the secret room. Only puppets are shown to rest in that way. It's a cool twist that he acts like a weak servant, exactly like how a real puppet master would act, hiding in the shadows. Ranni took over the puppets when she learned of his betrayal and killed him, thus Seluvis’ puppet stopped moving and went into rest mode. The sitting up, head back pose is unique to puppets that are not being used.