Dear Neville,
Which is ‘the Ashkenazi Mesorah’ I have five different candidates. Not including Lita.
Again, Lithunia was very unique. The spiritual environment and the living conditions were much different than the rest of Europe. Lita was not part of the Four Lands. The Rabbonim had more control over their communities than anywhere else in Ashkenaz.
If laypeople just make changes to the nusach it doesn’t go anywhere. It needs to be someone influential, or at least a printer. (But then the consumers have to not protest.) Do you think that the Shelah could change the nusach but not The Baal Hatanya? It would really depend on how much changes were being made in their days. My theory is that the fight over Nusach Ari is what stopped the constant changes. But a close observer will tell you that these things are still changing.
As far as I know, those minhagim where more regional, and are not specific to Chabad. When we discuss Chabad, Litvish and Ashkenaz differences matter. Because Chabad was more aligned with Lita. Most of the other Rebbes were Polish and not Litvish and were more like the rest of Ashkenaz in their locale.