Itchesrulik:
Point by point response.
1. All siddurim were copies of previous ones, though they were made by publishers, not rabbonim themselves who sought to codify the nusach. In that effort, Heidenheim was the one who took the lead, both historically as well as practically in publishing his siddur. What do you know about Heidenheim later in life regarding being other than a talmid of R’ Nosson Adler? Please provide references. The Chasam Sofer references him often with great kavod.
2. When I refer to “popular” nuschaos, I refer to the common, standard publications of siddurim used in most shuls. The chassidishe nuschaos suggested here are still based on choices made by publishers, who might be nice people, but not necessarily those who I would seek to be koveya a nusach. Artscroll used Heidenheim for reference, but so did many of the earlier published siddurim of Nusach Ashkenaz.
3. As far as kaballah, I do not make specific recommendation about that. All I noted, based on tshuvos from the Chasam Sofer and the Divrei Chaim is that a nusach that is based on the kaballah from the Ari is considered a nusach that is inclusive – it contains all the elements that the specific nuschaos of each shevet had. That is why the Divrei Chaim paskened that one may switch from Ashkenaz to a Nusach Ha’Ari. The logic, as I gather, is that this is not considered a switch away from the original nusach. I also acknowledged the greatness in kaballah of the Gr”a (who needs no haskomoh from me).