I'll leave it to the people you're accusing of being pretentious and making stuff up to respond to your final sentence, but...Great Vowel Shift. 😏 Mohder-mother-etc (that's why "mother" is written with an "o" but not pronounced with one. Large parts of the UK retained the original vowel while others changed.
Thank you for the Vowel O history lesson. 😁
Listen, I'm not intentionally being an asshole.
But honestly, and it's ignorant but true, I don't know a lot about other accents throughout the country, other than the basics. As an American, I just see 'mom' and wonder, from my London perch, why I would see an Americanism popping up in a culture where, in my mind, mum is the norm. It's my observation, an ignorant one too, I do know this. And now I've got your angle on things so, thanks for that. I should probably get out more and see my adopted country!
You make an interesting point though. My mother, who is actually from Ireland, went to the States in the '60s and stayed, spoke a lot about our American accents and dialects. She said that the New England accents (which all vary; Maine is a bit different than a New York or Boston or Providence accent, for example, but a thread runs through them, binding them together as 'similar') were more like the way British accents used to sound in Elizabethan times. Could be total hogwash and probably is, but it's interesting to think about. Our accents change so much due to time and influence.