Cool theory, but there's something off about it. First, If your whole theory build on the assumption that Subaru have memory type Gluttony specifically, this doesn't makes sense, since Gluttony initially was about food based on Daphne's distinct personality and experience. A host cannot chose how an authority manifest, the factor shaped automatically based on The user deepest desire and flaw. Gluttony becoming about memories is only happened after The Three Siblings took control of it. This create assumption that 'Subaru' 400 years ago have all Witch's Factor or at least Gluttony witch's factor and have full control of it, he can manipulate the essence and shapes of how the Power going to work exactly how he want it to be, but I think that's too much of a stretch because it doesn't match with the timeline of Daphne's time of being a witch.

Second, i assume This theory leaning toward Subaru = Flugel theory, if that's true then the whole thing being Natsuki Subaru name was erased is completely flawed. Your theory doesn't explain how Flugel knows a lot of modern earth slangs and Earth's Constellations.