This is cute, but it's wrong:
"Y'all" is a plural form of "you." That's it. It makes no difference how many "yous" I'm referring to.
Example: "You Brits? Well, y'all are are always spelling things funny. Like 'colour' or 'Kimberley.'"
See how I just lumped millions of people into a y'all?
"All y'all" is a plural plural and is used when there might be y'all subsets within a population.
Example: I'm in a classroom with 30 students. They are divided into two groups for homework assignments.
"Group A -- y'all read chapter four and write an outline. Group B -- y'all read chapter five and do the outline. All y'all need to be ready to discuss chapter six tomorrow."
Fifteen students will outline chapter four. Fifteen students will outline chapter five. All thirty students will be ready to discuss chapter six.
The moral of the story: Don't trust non-native speakers to correct your grammar.