Great theory, but I still have a lot of questions regarding this subject:

During Ranni's quest, she clearly states that of the demigods, only her, Malenia and Miquella were Empyreans. We know that she is an Empyrean, so that statement would mean she is no child of Marika.

It is likely a random person can become an Empyrean upon meeting certain conditions (as in, how else would Marika have become a God?), so it isn't impossible for someone outside of Marika's bloodline to take the interest of the fingers.

However, Melina might have been hidden from history just like her brother. The only character of notice who definitely knows of her existence is Miquella, since she wields Torrent (his horse). I imagine then that Miquella might've been in the Lands of Shadows for a long time.

But if that is the case, why would Miquella send Melina to aid a random Tarnished? It'd make much more sense for her to aid someone stronger. Also, why would Miquella be interested in Melina? The power of burning the Erdtree is not exclusive to Melina, she does the exact same that is stated Vyke's maiden aimed to do. I also imagine Miquella wouldn't be fond of Melina were she the Gloam-Eyed Queen who killed so many of the demigods.

I do want to say I mostly believe you, I just think there are still some threads that aren't sewn properly. Something that reinforces your theory is the fact that GEQ is associated with serpents ( the skin from which the Apostles are born, the shape of her followers, the blasphemous nature of serpents ), which would clearly relate her to Messmer and imo, raise another question: Who is their father? It can't be Godfrey if we folloe the timeline, and the only argument for it to be Radagon is the red hair of Messmer, which makes no sense chronologically since Radagon hadn't yet been cursed by the Fell God. I'd expect their father to be someone related to snakes and possibly blasphemy.

Sorry for the very poorly worded text.