I'm just saying I think constructions like "skeeters am a-humming" probably are pretty authentic for the dialect in question (although I don't doubt that a lot of slave dialect was kind of forced onto them by the 'massas', in keeping with their not being allowed to learn to read and write).   I think other aspects of the way these "dialect" songs were written, such as
"in de canebrake wid his little dog and gun" or "fo' yo' breakfast when yo' sleepin' time" or
"Bogie man'll ketch yo' sure unless yo' close yo' eyes" does seem to exaggerate the way some of these words were probably pronounced.

Looking at these lyrics, I notice that SOME words are spelled in a sort of exaggerated slang/dialect way while the spelling of others suggests quite "standard" English:

KENTUCKY BABE
Words, Richard Henry Buck. Music, Adam Geibel.
Boston: White-Smith Music Publishing Co., 1897.

1. Skeeters am a-hummin' in de honeysuckle vine.
Sleep, Kentucky babe!
Sandman am a-comin' to dis little coon of mine.
Sleep, Kentucky babe!
Silv'ry moon am shinin' in de heabens up above.
Bobolink am pinin' fo' his little lady love.
You is mighty lucky,
Babe of old Kentucky.
Close yo' eyes in sleep.

CHORUS: Fly away. (Fly away.) Fly away, Kentucky babe. Fly away to rest.
Fly away. (Fly away.) Lay yo' kinky, woolly head on yo' mammy's breast.
Um -- Um -- Close yo' eyes in sleep.

"Daddy's in de canebrake wid his little dog and gun.
Sleep, Kentucky babe!"
Why not "dawg an' gun" or "Kainrtucky?" too.

"Possum fo' yo' breakfast when yo' sleepin' time is done.
...
Bogie man'll ketch yo' sure unless yo' close yo' eyes,"

Why not "ketch ya when yo' sleepin' tahm be done" or "ketch ya sho' 'nless ya close yo' ahs?"

"Waitin' jes' outside de doo' to take yo' by surprise."

I think "Waitin' jes' outsahd de do' ta take ya bah supprahs" might be a better 'phonetic' representation of what the dialect/accent actually sounded like.

And would someone who said things like "Bes' be keepin shady" and "Close yo' eyes" be likely to say "little" instead of "li'l?"

This is what I mean when I say that accents are represented in lyrics by using ordinary letters to indicate phonetic sounds it's often unclear and inconsistent.