I grew up deep in "y'all" country, with well educated parents who made it a point to resist the local accent they felt sounded less educated. (It's not just the UK side of the pond differentiating between Eliza Doolittle and Henry Higgins.) They, and we and our friends, considered "y'all" to be a contraction of "you all" or "all of you" and therefore always plural; when we wanted to be specifically most proper, we said "all of you" and in colloquial (I do not know if any of us were ever actually cool enough to be cooloquial) went more or less through the motions of saying "you all" which of course came out in ordinary speech as "y'all" anyway. I now live in a part of the country in which this plural for "you" is not used and considered rather comic, so make more of a point of saying "you all" or "all of you" because "you" is still indefinite in many contexts. I do not recall ever hearing "y'all" being used to mean one person, but of course if one is speaking to, say, a friend asking him to bring his family to the picnic, one would say "I hope y'all can come" to indicate the family was invited, not just "you" the individual. It is funny, isn't it, how we consider there must be a correct or proper way of using what we all agree to be an informal colloquial term?
By the way, my family refers to the typo or misspelled word that turns out to be better or more descriptive than the original as a "bonapropism" obviously in contradistinction to a malapropism. I have a friend who is a master at this. (Writing about a songwriter who is almost as impressive as a well-known Scot turned Australian: "His songs Bogle the mind.") I do admire "cooloquial."