NCB:
The omissions of the GRA and Baal Hatanya were not necessarily because of the forgeries, though some were. I can cite specifics if you wish. More importantly, they were both extremely entrenched in Kaballah, and they applied their Torah knowledge to nusach.
The reality is that the prevailing Nusach Ashkenaz is considerably closer to “most accurate” than the common Nusach Sfard. It is more directly based on Rav Heidenheim’s siddur, and is much closer to the Ashkenazic foundations in halacha. The Ari Zal was far closer to a Kabbalistic based nusach, and there are variations of the mesorah from the Ari Zal among various talmidim of his. Many of his established nuschaos were codified By Rav Chaim Vital, his most prolific talmid. The Baal Shem Tov adopted many of these mesoros of the Ari Zal, and these are seen in the common Nusach Sfard. Tragically, the fact that these were not codified properly, there were different mesoros, and hence the variations seen in minhagim of different chassidim who recite Nusach Sfard with specific additions or omissions of certain words. Publishers followed this chaos, and each rendered decisions of what to publish in their siddurim.
One can access a huge number of Siddurei Ha’Ari Zal, whether in seforim stores or online. The variations are interesting, and many instances of these are based on different kavanos al pi kabbalah.
As for the “most accurate” nusach, the only reaction to that is to redirect the concern to the kavanah involved in davening. HKB”H will understand what we say, even if words are omitted or mispronounced, much as a father understands his child whose speech is not fully developed. It is wise to spend time studying siddur to have a better grasp of the content of tefilah, so that our experience in standing in the presence of the Shechinah is a wholesome, emotional process, as opposed to the completion of a chore that is required to move on to whatever is next in the schedule.