The way “rules” are derived now is through tests of native speakers. If the majority agrees on one way, that’s the de-facto rule.
Example:
The group of doctors is here.
The group of doctors are here.
Whichever one the majority of native speakers would accept as correct would become the “real” rule, regardless of what the books say.
Wait. What? In so much as there is a universal style guide in the US, it’s the Chicago Manual of Style, followed by the Associated Press Stylebook. APA and MLA are fairly academic style guides. Even the AP Stylebook is generally only used in the news and magazine industires (but, as those comprise a great volume of the written word, the influence should be noted.) If you had to choose the most widely used, general US style guide, it would have to be the Chicago Manual of Style.
It’s not that simple.
Language change doesn’t happen simply because a majority of users say so. The majority has to be that of “good” writers. As I have often before, I define “good” writers as working professionals: journalists, authors, teachers, all the people that use words regularly and for an audience.
We know that mere majority usage doesn’t necessarily become standard. Ain’t is still substandard after all these years. The use of apostrophe’s in plural’s is seen in zillion’s of place’s, yet is still totally derided.
And calling usage standards “grammar rules” as you do in post #11 is still wrong, no matter how many threads are started by OPs making that mistake. Because of that mistake you make an additional one. The books by definition can’t be “wrong.” They are style guides, not rulebooks. They can and often do contradict one another. They are not intended to make usage right or wrong but to provide a consistent standard for writers, editors, and copyeditors to follow. Unless you are required to do so by a higher authority (i.e., one that’s paying you money) you can follow these guides or ignore them and nobody will care.
There are no “grammar associations” in the U.S. Slang is not grammar. Style guides do not even contain standards as to what slang is proper or not. Usage is not style either. I don’t think you understand what a style guide is to begin with. You certainly need to stop thinking of them as having "rules. "