Having never given much thought to it, your analysis rings true to my native Texan ears.
There's another usage that comes to mind, though. One might argue that "y'all" borders on a second person plural inclusive of the speaker whereas "all y'all" marks a distinction between the speaker and the others. For instance, a peeved person would be more likely to say, "All y'all can kiss my ass," as opposed to, "Y'all can kiss my ass." "Y'all" by itself is more friendly and self-inclusive than "all y'all", which carries an inherent otherness to it.
I knew I explained that poorly. What I mean is that, in comparing "y'all" to "all y'all", a simple "y'all" is "you guys (and maybe me)" while "all y'all" is "you guys and not me".
Grammatical constructs can have a lot of variation between languages, and there are certainly nuances that can't be expressed in English the same way that it can be in other languages. One thing we lack is a nuanced sense of past, while other languages have baked in ways to express recent past or distant past (e.g. Bantu languages).
My proposed interpretation regarding "all y'all" is not academic, just a native feel, and I'm sure other native speakers could disagree.
Hmm, interesting. I'm not from Texas but I have family who is. I'll listen for this one also.
I'm under the impression that the double negative is a relatively modern thing (early 1800's). Previously, repetition of the negative just reinforced it, like:
> I ain't never put syrup on my bacon on purpose
...just double-enphasized the negative, rather than letting the second negative negate the first. This feels similar except instead of stacking negations you're stacking separations.
There's another usage that comes to mind, though. One might argue that "y'all" borders on a second person plural inclusive of the speaker whereas "all y'all" marks a distinction between the speaker and the others. For instance, a peeved person would be more likely to say, "All y'all can kiss my ass," as opposed to, "Y'all can kiss my ass." "Y'all" by itself is more friendly and self-inclusive than "all y'all", which carries an inherent otherness to it.