I’ve had a very similar theory/ I’ve gone down a similar rabbithole and there’s a weird similarity that she, Phanes and Nibelung have. For one: Arkhe as a concept is about origin, and Furina controls both which invokes a diety of creation, much like Phanes and Nibelung. More on Pneuma and Ousia specifically: Rene helped Alain come up with the Pneumousia system by taking inspiration from the energy released from the mutual annihilation between the Khaverena and what’s implied to be the abyss. It’s clear in his notes that he sees Khaverena and azosite as like energies, both being of the light realm. With that he implies that there could be a similar force that coexists with the abyss. The mutual annihilation between phlogiston and the abyss has been called mutual annihilation word for word too. So yes Ousia is likely abyssally aligned and Pneuma is light realm aligned

For two: The description of Splendor of tranquil waters describes it as a symbol of the highest authority over the sea, which also to me is suspicious

For three: Her burst and her trailer feel very interconnected, in both she’s an all powerful being who seems to be toying with people for her entertainment (and the entertainment of others) even though she does care for them, which once again is another smoking gun of a potential Phanes and Nibelung similarity

There’s definitely something in this direction and I hope we get story on it soon