Ah, Hal's correct..."all y'all" is indeed plural, whilst "y'all" is collective. I still stand by my assertion that "y'all" is never to be used to refer to only one person, however. :^)
Agreed. Singular person is still "you".
"Y'all" is reserved for the collective.
"Y'all's" is possesive for the collective.
Using "y'alls" as plural is just bad grammar in these parts.