How can one deal with obsessive thoughts that involve insulting the Divine and engaging in sexual acts with God? And could these obsessions lead to polytheism and disbelief?

What you are mentioning is a type of obsessive-compulsive disorder for which there is no accountability. It is treated by completely ignoring it, seeking refuge from Satan, occupying oneself with learning, productive work, and permissible entertainment, and avoiding isolation. One can also consult psychiatrists and seek ruqyah (Islamic exorcism).