A commenter on the Dead Sea Scrolls post remarked that a variant in the scrolls seems to point to polytheism and that "critical scholars regard the text as having been edited to write out the polytheism." I have a problem with conspiracy-type claims in general because they usually end up being based on what's not there rather than what is. Speculating about what was removed--something we don't see now--is a shaky enterprise.
And, even worse, in order to believe this theory in the case of the Bible, one would have to believe that not only did the pro-monotheist conspirators remove all references to polytheism, but they also added numerous passages and changed the entire thrust of the Old Testament, throughout which God declares Himself to be the only real God--the One who created everything, who won't share His glory with anyone else, who prevails in every nation, in every setting, who owns everything, etc. Consider the variant in question. The claim has been made that it could be interpreted as describing a high god dividing up all the people of the world and giving some to Yahweh and the rest to other gods (I give an alternate understanding here). But again, look at what is actually there in the larger context. Throughout the Old Testament, God is angry with people for serving false gods--even those who aren't Israelites. How would that interpretation make sense if the high god gave the people of other nations to those other gods? Why would God be angry with people for serving the gods to whom they were given?
We're not talking about a few, isolated monotheistic passages here that need to be explained--the Israelites struggle against polytheism throughout the entire Old Testament and are frequently punished for giving into it. And why are they punished? Not because they are worshipping the wrong god for them or even merely because they have broken some rules. God charges them repeatedly with failing to show the truth to the world that He is God. Even as early as the Exodus, God acts with the express purpose of showing the world He is God. Monotheism is central to every theme of the Bible. Does it seem likely a few people changed the Old Testament so completely and got everyone to go along with it?
In fact, if you look at the reasoning used to prove early polytheism, you'll find that the assumption is made before the evidence is examined that the early Jews were polytheists. Here's the reasoning: Monotheism evolved out of polytheism (base assumption). Since all the nations began as polytheists, so must the Jews have. Therefore, the Jewish idea of monotheism must have evolved over time and been inserted into the Bible later. All the conspiracy is determined from there as they look for anything that can be construed as a remaining trace of polytheism. When they don't find the evidence they expect to find (based on their assumption of the evolution of religious thought and the falsity of the Bible), they then use that lack of evidence as evidence for a conspiracy (i.e., the polytheism that should be there was removed). It's circular. The missing evidence is not proof of a conspiracy, it's justification for their prior assumption. (And in fact, there seems to be some evidence supporting the opposite base idea that polytheism evolved out of monotheism.)
Do you see how one has to begin with the assumption that religious beliefs are both evolving and man-made in order to come to this conclusion? If, on the other hand, there is a one, true God who revealed Himself to a group of people, it's not inconceivable that that group of people could believe in monotheism even if the other nations around them believed in polytheism. In that case, we would expect to find in the Bible what we do actually find.
(You can read more about the religious evolution theory and responses here.)