“The little I know” seems a little inaccurate as a screen name, no?
I have a couple comments about what you said:
1- You call R’ Heidenheim “the first one to publish the Ashkenaz siddur with any attention to true nusach.” He was the first one to publish one with attention to reconstructing a true nusach. All siddurim try to be accurate. Also, he was anything but a talmid of Rav Nosson Adler later in his life.
2- Which nussach sfard do you mean when you say the “popular” one? Some of them are chassidishe Nuschaos, others, like artscroll are based on R’ Hadenheim
3- Why do you say it is necessary to use nuschaos that are based on kaballa?