The background to the proliferation of nuschaos is that each shevet had its own nusach that corresponded with its individual derech of avodas Hashem. These nuschaos were lost with the golus, and the prevalent nusach hatefillo was established in each of the major communities, Ashkenaz (Germany and neighboring lands), and Sforad (Spain and neighboring lands, including North Africa). These predate the printing press, so that the central establishment of anything was a fixed thing, though the hand copying of all texts meant that human error and modifications were common. I do not know enough about the nuschaos of Sfard, but the first one to publish the Ashkenaz siddur with any attention to true nusach was R’ Volf Heidenheim, who was a talmid of R’ Nosson Adler, and a chaver to the Chasam Sofer. He published his siddur 8 times, with changes in each printing, as he became aware of nuances that required change.
There was a sefer on tefilloh that was printed in Berlin in 5545 by Yitzchok Stanov (Vaye’etar Yitzchok) in which he proposed various pshotim on tefilloh together with changes in Nusach. A study of history reveals that this fellow, albeit knowledgeable, was a maskil, and far to the left even among maskilim. Despite this status, many of the changes he made became popular and incorporated into the siddur. As R’ Volf Heidenheim became aware of this source as the background for certain aspects of nusach, he changed them back in the next printing of his siddur.
Today’s Ashkenaz is based primarily on R’ Volf Heidenheim. Much of the popular Nusach Sfard is similarly based on Heidenheim.
Bottom line is that today’s popular siddurim, both Ashkenaz and Sfard, lack the exactness that should really be found in the siddurim we all use (though Ashkenaz fares better than Sfard). In this climate, it is worthwhile to address nusach as something that has been established by someone with the knowledge in higher realms. In the Chassidishe world, this would involve any of several mesoros on nusach from the Ari. In the Litvishe world, one should examine the siddur from the Vilna Gaon who also attended to nusach from the realms of halacha and kabbala.