I can't offer a consensus on your question about the parentage of Roxy's
child, but I can offer you my opinion, FWIW.  I agree that the logical
father of the baby is Driscoll rather than Essex, and that Mark Twain was
blurring the lines to avoid added controversy about miscegenation.  My
"evidence":  the two babies are born on the same day, one to Driscoll's
wife, one to Roxy.  They look identical--and having the same father would
make that more plausible. Did Driscoll couple with both his wife and Roxy on
the same day?  That would be a sickening possibility.  Metaphorically, Tom
and Chambers are the "extraordinary twins," separated by Twain by blurring
the fatherhood question.

On a somewhat related note, and my favorite footnote in my book __Mark Twain
and Metaphor__ (shameless plug:  Missouri, 2007), is Roxy's racial heritage.
If she is one-sixteenth black, that would mean she is the product of a long
line of miscegenation, all presumably resulting from her fully black
great-great grandmother's coupling with a white male, with each generation's
subsequent child female, and each later coupled with a white male (I am
assuming that black male/white female relations were extremely rare).
Tracing her lineage shows a nightmare of miscegenation that Twain muted but
laid before us if we think back (fictionally surmising).  Five generations
of racial rape...no wonder he had to mute things.

John Bird
Winthrop University