In many cases, it's okay. Reading manga can be a form of entertainment and relaxation. As long as it doesn't interfere with his responsibilities or your relationship, there shouldn't be a problem.
No, it's not okay. Spanking a baby is not a proper form of discipline at all. Babies are too young to understand such actions as discipline, and it can cause harm to them both physically and psychologically.
Yes, it is. But you need to make sure it fits well and doesn't feel forced. Also, be careful about copyright issues if you're using specific elements from the movie.
No, it is never okay to cheat on your husband. Cheating is a betrayal of trust in a relationship. It can cause a great deal of pain and damage not only to your partner but also to yourself. If you are having issues in your marriage, it is better to communicate and try to work through them rather than taking the unethical path of cheating.
It is completely unacceptable. In a healthy marriage, communication and openness are key. If you want to know something about your wife, you should have an honest conversation with her rather than spying on her in such a sneaky way.