re "mom/mum" - According to this page, it's a linguistic fossil, just as with "herb/herb". "Mom" and "'erb" were once common in parts of England whence a lot of folks emigrated to the US. The English dialect shifted - the American did not, at least not in the same direction. We were too busy overpronouncing our "r's" (except in the south) and constricting our "a's" to pick up on "mum" and "herbs."
gently feral