You appear to be saying that y'all is formal singular as well as plural. An extensive discussion on this page shows clearly that people do not agree what y'all means. But if that is what you are saying then that is fine because sibh also covers the formal singular. However I have not previously heard anyone argue that y'all can be formal singular in preference to a non-formal singular. Further, there seems to be such a lack of agreement about what it does mean that it is not obvious how they could accept it on this course.
JaneyCKidd, I'm not quite sure of the point you're making. Although not stated clearly on this page, the discussion I link to above clearly shows we are talking about American dialects, and I would be surprised if anyone thought it was other than American. As for it being Southern, the passage I quote in the link shows it is thought to be expanding beyond the dialect it started in when it says
Due to a cultural shift in the United States by non-Southerners using the word, it is now rarely also used as a singular you, although most (increasing) non-Southern / non-AAVE use is, like Southern and AAVE use, plural.
Note that 'AAVE' is African-American Vernacular English. This is not a dialect that is restricted to the South, so that implies it has already spread geographically.
sibh, sounds very different when spoken in the sentence to when you mouse over it and hear it on its own. One sounds like "Shive" and the other sounds like "Shoe".
I'd really like a Gaelic cheat sheet where it chopped up the various letter combos into sounds like XXX to aid reading and then speaking it.