Translation:How are you guys?
"Y'all" is a contraction of "You all", so "all y'all" would be "all you all" which I think is a bit redundant.
"Y'all" means "you all" but when there is a lot of you all you say "all y'all", which means "all you all"
Are you saying that a valid translation is how all y'all doing? Cos that sounds odd. What Dan_ri suggested sounds better.
Granted. If you were an interpreter translating for an audience of English speakers from various backgrounds and countries it would probably be wise to adjust your translation to suit the understanding of a wider audience though.
It's regional. "You guys" for everyone is common in some but obviously not all areas.
The pronunciation of みんな seems almost indistinguishable from みな. They must be more different in actual speech than I understand them to be.
It's also interesting that the longer one is considered more colloquial.
In English, that sounds to me like something bad happened and you're checking on them. I think 大丈夫/だじょうぶ would be better for this than 元気/げんき.
Also note that this is very informal so please don't use this with people you don't know and who aren't your friends as it could come off as being rude. Use the more polite version of みんな、元気ですか。
Phonologically speaking, is the change from Mina-san to Minna (with the extra N, like a stop) similar to how for example Kochira becomes Kocchi (again with a stop)? I mean in the casual form the word shortens and a stop is added. Or is it just a coincidence?
This is too colloquial, and unlikely to be answered correctly. Please change this to "how are you all"