If demons have free will, can a demon ever turn back and follow God again?

No. That is the punishment, and it shows you how severe God is in his punishments. The devil became the devil in the Garden of Eden, because he saw it as his one chance to trap God so that there was no way out and the devil would rule the world with God put to the side. He deceived at least a third of the angels to believe they could trap God Almighty, except they never realized that God would humble himself so much as to become a man to take care of the problem; that never crossed the devil's mind. For that kind of sin, so deliberate, so calculated, so in-your-face to God, there is no forgiveness; God sees to it that the punishment says you will live forever in this state. For man it was different, because he was really deceived, first by the devil, then through the woman, and did not premeditate it the way the angels did, so God gave mankind a second chance, which required that he come as a man to deal with the sin problem. So to answer your question, no, there is no chance for that, none.