Hamlet's feigned madness is a device that he uses to stall and give him time to figure out what to do about his Uncle having killed his father and married his mother. Hamlet's madness is contrived while Ophelia's is real because Hamlet has a strong mind and Ophelia does not. Ophelia is torn between father and boyfriend when both manipulate her in order to get to the other one. Ophelia is not one of independency or strong will. She follows the dictates of society which tell her to obey her father without question, but also to to trust everything that she sees. Had she used a bit of woman's intuition and investigated on her own without daddy looking over her shoulder, she probably would have put two and two together before losing her mind. This is part of the answer to the second question, too. Both Hamlet and Ophelia knew that daddy was spying the whole time, so neither one could be themselves and have an honest conversation.