OK! Here we go.

1.) How do we know that the manuscripts aren't forgeries?

well, we know for sure that the people I named are the people who wrote these manuscripts out. we've got lots of samples of these people's handwriting. They were both prolific copiers for Bach - Anna Magdalena as a way to help out her Husband who had a ton of jobs, and Kellner because he was a big fan and fellow keyboardist.

With that in mind, it's pretty clear that they are what they say they are on the tin.

2.) there HAVE been forgeries, though.

I wrote an essay about this, actually. there's this dude from the turn of the century named Henri Casadesus and he (with his brother Marius) are infamous for forging pieces. most famous are a Mozart Violin sonata and a J.C. Bach (J.S. Bach's son) Viola Concerto.

I actually have a pet theory that Marius and Henri didn't even write this music. I figured out that someone (most likely Henri) copied some pieces by Luigi Borghi, arranged them as a sonata for viola and Bass, and marketed as by G.B. Borghi. It's possible that the Casadesus brothers did the same with the Mozart and J.C. Bach.

3.) Two videos for you:

and alternatively

4.) Well, in my opinion, it hasn't - for example:

and

same composer, different editions. which one is the "correct" edition. Should you try to play it exactly like either of them? if you didn't, you would create your own "edition", right?

what if your edition of an earlier work becomes more known and more popular than the original?

( it's possible that this is not even a Leadbelly tune, and may indeed be older).

5.) see 3.

6.) no, i didn't. but i think it's because even though there's no questin mark, it still asks a question in its way.