ChofetzChaim. Brilliant post. However, two comments:
1) There is a mishna in Eduyos which says Chayav odom lomar bloshon rabo (I believe.) The rebbe there had a speech impediment and mispronounced certain words. Nevertheless, the talmid was required to pronounce them the same way. (Don’t have in front of me, and too tired to look up.)
2) I am extremely troubled by the quote from the Yesod vShoresh Haavodah that the holocaust and other tragedies are caused by something as minor as mispronounced words. How would he have known? Doesn’t the RBSH understand our tefilos even if not perfect. One can even daven in another language, altogether. (I am aware that the Tosfos Yom Tov is said to have had a dream that Tach vTat was caused by talking in shul. I am just as troubled, and furthermore, how can we learn from mayseh chalomos to denigrate the holy klal yisroel, when Chazal say Ein cholom blo dvarim bteilim.