Hmm...
Yeah, I think I am also gonna join in on the club of readers who theorize that there never was an "OG" Miyu.
The fake Miyu may just be portraying an idealized version of OG Miyu, and the OG's true self may have been completely different from what everyone expected of her.
Who knows, maybe OG Miyu not being as everyone wanted her to be WAS what led to her death somehow?
I am open to other theories too.