Now, Metal, play nice. Your particular example fails to account for the differences in vernaculars. There are more English vernaculars spoken than there are English speaking countries. In my home town, the example you quoted is most generally rendered, "Ja-et-yet?" As we all know, spoken vernacular and written language are not the same thing at all, at all.
I don't think it's anything to do with vernaculars, or do British English vernaculars also use the past simple instead of the present perfect there?
According to dictionary.com, one of the definitions of vernacular is: "the plain variety of language in everyday use by ordinary people". This is exactly what I mean by my remarks concerning vercacular, and yes, I believe it has a lot to do with why people speak as they do, be they British, American, Indian or what-have-you.
Still, I do understand your reaction. I have the same reaction whenever I hear British friends talk about what they did "at" the weekend, or when they replace initial /th/ with /f/.
That's because British English speakers see the weekend as a point and Americans see it as a period. I can't see why Americans use the past simple here though:
Did you see the new movie at the moviehouse (yet)?
Hi. Did you eat yet?
Could you explain why you lot choose the past simple there?
Okay, you obviously concede that choice of preposition is determined by one's perspective or concept. I submit that in your examples, the American concept of finished time is not the same as the British concept. Most Americans do not view eating or seeing a movie as an action that began in the past with results that continue into the present. Eating is an action. It was done a few hours ago. It's over. I ate. That's it. I will eat again, but I'll not continue eating any one meal indefinitely. So, simple past, in both questions and answers, "feels right". However, we will use the present perfect if we want to
emphasize that we have already eaten, so please don't pester us about anther meal just yet, thank you.
In most American vernaculars, the use of "shall" is either fading or dead.
Yes, as I said, American English tends to conflate forms which could be usefully kept apart.
I disagree with your concept of confaltion. In these cases, the forms are not really conflated. One form is simply dropped in favor of another. It's called the evolution of a living language. I'm sorry if it offends you, but there's not really anything you or anyone else can do about it.