ujm:
The chassidim intentionally corrupted the proper pronunciation as a counter-force to the maskilim. Presumably, this was an “eis laAsos”-type decision by their esteemed leaders. Unfortunately, they have not switched back, even though those maskilim (and their danger) are gone.
As mentioned, with the added exception of those whose havara is corrupted by yiddish (or by a real language), everyone does milEil/milRa the same way, as it should be done, if I am not mistaken.
Regarding the other differences, like many sefardim pronouncing kamatz as patach, it is interesting to note that Persians, who have a common “kamatz” sound in their secular language (Persian), do distinguish between kamatz and patach. So, it is obviously a cultural (i.e., Arabic language) influence that corrupted the general sefardic havara to lose the distinction between kamatz and patach (and the same idea by Ashkenazim who do not distinguish between alef and ayin).
Teimanim have the most number of distinct letter sounds of all, including our English “th” sound – which makes sense as their mesorah was intact from after Bayis Rishon until the Zionists viciously shmaded them – but they also seem to be influenced by their native culture in their not distinguishing between kamatz/patach, if I am not mistaken.
But, again, the point is that Lashon HaKodesh absolutely does not evolve, as G-d made that exactly as He wanted.