I've seen it argued that Judaism was probably polytheistic before they were conquered by the Babylonians. How aware were Jews of this by the time Jesus lived and studied?
The Bible is replete with stories of wayward Jews who worshiped foreign deities, but the Bible severely condemns this behavior.
The Judaism of the Bible is monotheistic (not monolatristic or polytheistic) and denies that those deities are real. See, for example, Deuteronomy 4:35:
Unto thee it was shown, that thou mightest know that the LORD, He is God; there is none else beside Him.
Know this day, and lay it to thy heart, that the LORD, He is God in heaven above and upon the earth beneath; there is none else.
Thus saith the LORD, the King of Israel, and his Redeemer the LORD of hosts: I am the first, and I am the last, and beside Me there is no God.
There are plenty of verses referring to other gods, but the context of the above cited verses indicates that Judaism only accepted that those gods existed within the framework of the mythologies of other religions. Judaism did not accept that those deities were real.