Did I miss something, this is based largely off her handwriting.. presuming it's accurate and it's not his hand writing. Could this not be something as patently simple as, he played, she put it down on paper, something I'm led to understand isn't exactly unheard of.
Nor is having an assistant and a relationship growing between people turning into a marriage. Who knows, I don't care either way but basing an entire theory around things being corrected and things written in her handwriting and jumping to the conclusion she composed the piece is... well a logical leap to far for me.
She's known to have transcribed things for him at other times but....
While Anna is known to have transcribed for Bach in his later years, researchers found the handwriting did not have the “slowness or heaviness” usually attributed to someone who is merely copying, but was likely to have flowed from her own mind.
flim flam
Honestly sounds more like someone twisting the evidence to suit their own agenda than having any real proof.
So it's known she transcribed some stuff for him, but her handwriting on other pieces is proof she did the composing because... she wrote faster? Maybe it was, you know, another copy because the first one was messy

I've done that with maths proofs more times than I care to remember.
ultimately off what is basically no evidence and with no one who actually knows then honestly I think this is one of those things that is a complete waste of time.
There are times in history and places on earth today where the idea that a woman can compose beautiful music would be scorned. TO suggest that a woman in a lets say western or educated country that values women's work will suddenly have the confidence to get into music because it might turn out that some of Bach's work was written by a woman.... really?