A Cambridge study suggests that atheism was not necessarily caused by religion. In fact, the evidence at hand suggests that atheism appears to have been around as long as religion.
The OP, while offering some value, is also quite limited to the scope of the restorationist Protestant movements that developed from America's Second Great Awakening period. These groups, like the Seventh Day Adventists, the Mormons, and Jehovah's Witnesses, fit into the paradigm you suggest, but others do not.
Note, for example, your second point:
If Scriptures were immune to scientific errors. Judaism, Buddhism, and even early Christianity were not based upon Scriptures. Instead these movements would later produce and add religious texts to their already functioning religions. These texts did not act as the foundation of their doctrines as it does for the restoration movements. The idea that religion is based upon "proof texts" and a particular canon of scripture was developed by Marcion of Sinope of the 2nd century C.E. These new religious movements from the 18th and 19th centuries revived Marcionism and introduced the claim that not only should religion be based upon scripture but science and history as well.
For this point to be applicable all religions would have to start from a scripture text as well as claim that the texts are meant to be read as historically and scientifically inerrant. Judaism, for instance, has never advanced the idea that Genesis was a historical account.
The claim that religion must be based upon sacred texts is problematic as scripture texts don't compose themselves. They are the product of religious adherents, meaning that a religious writer must exist first. For this to be true a functioning religion with a set ritual or cult of worship would have had to exist to begin with in order for there to be a religious adherent who could compose such a text. The idea you espouse is based on religious movements that to a lesser or greater degree took scripture texts composed by other groups and made special claim to them. Their theology rests on the paradigm that would have to mean that scripture came before religions since the texts act as their basis (an impossibility).
Theism cannot be argued to be the natural course of all, even in ancient times. Therefore atheism's existence cannot be said to rely on the existence of theism.
