Oh come now, TRex. Where do you draw the line for the South?
Virginia (southern and western), both Carolinas, Kentucky, Tennessee, Georgia, Florida (northern), Alabama, Mississippi, Arkansas, Louisiana, and Texas (eastern). (Not Missouri.)
And I'm not a Chicagoan even if I live near it.
I grew up hearing "Y'all" referring to singular all the time. Go shopping and they call out "Y'all come back now, hear?"
I've only heard that from people trying to sound Southern.
The plural form is indeed "all y'all" and the plural possessive is all y'all's.
I have
never heard anyone use that combination.