Doesn't Satan make Bible-based mythology polytheistic?

As if having a Trinity and Satan and Angels wasn't enough, yes.

Well, first of all, Paul molded his version of Christianity to be more comfortable for polytheists to convert to. He gave them saints in place of gods, plus he 'borrowed' many customs, sacraments, holidays, etc. None of that goes back to Jesus. Jesus was never anything but Jewish, and Jews don't really believe in the Devil, and didn't then. Most of what Christians 'know' about the Devil came much later, in the Middle Ages, not in the Bible or Biblical times.

Secondly, what you are seeing here is a natural function of how the human mind works. Just as humans have always seemed to have a need to believe in something, they've also seemed to gravitate towards philosophies where there are two Gods, one good, one evil, and the universe is a battle between them. The very first monotheistic religion was Zoroasterianism, which had a very strong influence on Judaism and Christianity, which came later. Zoroasterianism had two deities, Ahura Mazda, the creator God, and Angra Mainyu, the evil destroyer god, the destructive spirit. Strangely, this duality is called a 'manichaean view', based on another religion, Manichaeanism, which came much later.

Christians seem to want to see the world as a battleground between good and evil, so there has to be someplace both come from. Personally I think evil is just the absence of good. Light and dark aren't two different things, or cold and hot, so neither is good and evil. But I see how the duality idea supports Christian beliefs.

Satan is not responsible for the contents of the Bible, just for the behaviour of some characters within and without the Bible.

No. Polytheistic means you worship more than one god. Satan is not a god.

No satan is not a god, he is a fallen angel that wants to be god. God is far above and beyond him. He can wish all he wants but he will never be God or even close.

As long as you are not one of them cornholers you should have a chance at getting to heaven. Good Luck and God Bless.

satan is not God. God and His word are real. Just because you mock and scorn does not negate spiritual reality.

Satan is not God

I was under the impression that teaching a parrot to pray was polly theistic.

Not really no