"You all" for "you" plural
This is really a question about the English being used in the Latin course, but does "you all" (not "y'all") actually suggest a simple plural "you" to most Americans? In the Plurals skill in particular it seems to be being used a lot to force a simple plural Latin phrase.
As a native Hiberno-English speaker with English influence, "you all" only suggests "all of you", which is quite a different connotation. I suppose it's possible Latin didn't make the distinction, but it seems unlikely.
I think it is inappropriate to use an expression like "you all" in the exercises to try to elicit a particular response. "You all " is not a standard part of English. It is part of southern US dialect, or it is slang (such as "you guys") There is absolutely no reason to distort English in order to practice Latin. If a plural response is desired we could very simply be asked for a plural response without mangling English.
It's regional U.S.: southern. They might have also chosen "youse" or "y'uns," which are used in other regions. It seems to be employed in an attempt to differentiate plural and singular you. They also might have opposed the archaic "thee" and "thou" to you to the same effect.
"You all" or "y'all" is annoying, IMHO, to say the least. They would do better just to accept that "you" can be singular or plural. If they insiste on using "you all," it would be much better if it were used consistently, which it is not. Hopefully this will be ironed out before Beta test is through.
I don't use 'you all' in my regular speech, but I actually think it was a good choice to include in the Latin course, as there is no english equivalent to a plural you. I wish French and Spanish courses would also mark when they want a plural answer. I find otherwise I tend to 'cheat' and only practice the singular forms I am more comfortable with. Or, maybe they could go with you (pl) in brackets, instead, to mark the plural, like some other language courses do.
What is the point of cheating, as you put it? Allowing either second person singular or plural answers allows us learners to practise whichever we feel less comfortable with. When the exercises allow it, I sometimes try alternating between using the singular and plural versions in successive lessons to make sure I'm practising both.
"You are" is already subject plural (acc), ye is object plural (dative)
Thou bist, beest is singular subject, thee is object.
They is subject, " 'em" is object.
I gave thee an apple, but thou hast given a pear.
"youse" is fine though. actually, it might help people if they use the more familiar pronouns.