I still dispute that American English has changed less than British English. History is written by the winners after all, so it shouldn't be surprising it's what you've been taught, given the fact the USA is a dominant cultural power. I have also informatively studied some social science, although not linguistics and I take some of it with a grain of salt, because it isn't a matter of right or wrong as is the case for hard sciences such as engineering. It isn't as though there are recordings from 250 years ago to prove one way or the other.
I resent the insinuation that I’m only saying that because of patriotism: as I already explained, this is the conclusion of
British historical linguists of the English language
in England.
And honestly, it shouldn’t even be surprising: England has far more deliberate pressures on diction, thanks to the much, much larger importance of socioeconomic class there. (Americans pretend they don’t have classes.)
Regarding the rest of your post: yep, one of the things that is very, very frustrating in layperson discussions of “American vs British” English is that examples of nonstandard (i.e. regional or other specific dialect, but also simple examples of errors) American English get compared to RP, ignoring the cornucopia of regional dialects in the UK and Ireland, most of which are scarcely closer to RP than General American dialect is! Many of those dialects retain historical features that American English also preserved.
And some of them were brought to USA as well. I don’t know as much formal history about it, but it hasn’t escaped my notice that one of the two (modern) native dialects of Dublin sounds
astoundingly close to (modern) American English. Given that the Irish came to USA in droves, later than the English, it stands to reason that their dialect(s) would have a palpable influence on American speech.