"In what's called "Broad Australian" (basically, a rural and working-class ... Present English which these other varieties are 'trying' to fill."
"The formal "you" (the second person plural like the the French "vous") replaced the familiar "thou" by the end of the 17th century."
This is a confused way of putting it. As I said, English had a 2nd person formal singular up to the eighteenth century, before the formal and informal pronouns coalesced in the neutral singular (and plural) pronoun "you".
"I don't think that the second person plural "youse" (Irish, Scottish and Australian) or "y'all" (South-Eastern USA) could be called formal by any stretch of the imagination. Regards, Einde O'Callaghan"
Fairly obviously I'm not using 'formal' in a colloquial sense here but to label a structural linguistic opposition. When a barber in N. Carolina says to his (singular) customer "Y'all come back now, y'hear?" he is clearly using "y'all" in the way the French use "vous", the Germans "Sie", and so on. .