The reason for the different nuschaos among Klal Yisrael seems to be simply different versions of the exact words of the tefillah that developed in different countries. If you look in sefarim on tefillah and halacha from the time of the Rishonim, you’ll see that there was never a unified nusach of tefillah (the differences between earlier nuschaos are much greater than the differences between modern-day nuschaos).
The Arizal said that there are 12 she’arim of tefillah corresponding to the 12 shevatim, and each person should daven the nusach of his shevet. He added that there is a 13th sha’ar which is for one who doesn’t know which shevet he’s descended from. The Ba’al Hatanya wrote a siddur in an attempt to identify this 13th sha’ar. That nusach is the so-called “Nusach Ari” davened by Chabad Chassidim to this day.The Arizal himself was an Ashkenazi, and at least on Yamim Nora’im davened with the Ashkenazim.
There seems to be no evidence that the Baal Shem Tov davened anything other than Nusach Ashkenaz, but his talmidim tried to daven according to the Arizal’s kabblistic kavvanos, many of which he (or R’ Chaim Vital) explained according to the nusach of the Sefardim. A cursory look at a few “Nusach Sefard” siddurim shows that no two are the same, but in general they all seem to combine the Ashenazi and the Sefardi nuschaos (sometimes creating absurd phrasing).
The chassidim met with a lot of opposition for changing from their ancestors’ nusach, and many of them defended it on various different grounds. Many teshuvos have been written from in the last 200 years on the topic. But I don’t think anyone is going to change their nusach based on anyone’s arguments either way.