1) it's y'all, not ya'll
2) its all y'all, not ya'll's or y'all's
3) the world is your oyster is a weird one for me, since I do not like oysters. But do love the pearls, so I guess I'll take the oyster, if it has a surprise in it.
That concludes your lesson of the day.
So, I have a question...what's up with your avatar and screen name. I am sorry to say that the fish face is staring at me and becoming annoying!...this observation is funny coming from my avatar, I know.
Being Southern to the core, I know of the many nuances of "y'all". Given that it's similar to "ain't", I don't know if there is a "right way" to spell it, but I'll bow to your grammatical interpretation.....excepting that I retain rights to the use of the plural "y'all's" at my discretion, since it demonstrates the possessive form of "y'all" more correctly IMHO than "all y'alls".
As for my avatar, I think that, of those available from the site (which I took on short-notice, over use of no avatar), Nemo represents my personality fairly well, though I will work on finding a less-offensive one to use for the long haul. And, after reading, and listening, to you and DD, I agree that all y'all's avatars seem to fit your respective personalities well.....no offense intended (I'm just agreeing with you).
Sunshine and happy thoughts to you from me....stay tuned, I'll work on my avatar over the upcoming days!