Nice job on this theory guys
Although I'm not too fond of the whole idea of BZ being Hamura, that does seem to be the direction that Kishi is taking. I liked the reference to Tobi and BZ's voices changing after they stopped acting.
One thing I'd like to point out is this scan
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. Here it states that the reason for Kaguya taking the form of the Juubi along with the Shinju was to take back the chakra which had been divided between Hamura and Hagoromo. So I am going to assume that parts of their chakra were necessary to fulfill whatever Kaguya's true plan is. In other words, she did not have enough. Recently, we saw Kaguya come to the conclusion that she would eliminate Naruto and Sasuke instead if trying to take back their chakra
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. We know that Naruto and Sasuke each have roughly half of Hagoromo's chakra in addition to that of Indra and Ashura.
So the question is, why would Hagoromo's chakra (now in Naruto and Sasuke) be so important prior to the sealing, but be disposable currently. If Hamura is indeed BZ working with Kaguya, then that means she already has access to the entirety of Hamura's chakra or at least a piece of it. This could be why she can still follow through with her plans even though she lacks all Hagoromo's chakra.