If I persist in the practice of masturbation—given that I suffer from obsessive-compulsive disorder, which generates blasphemous thoughts linked to masturbation—will I be held accountable for these thoughts, or only for the act of masturbation itself? And what if I were to marry while in this state?

You are not held accountable for intrusive thoughts unless they are accompanied by speech or action, and your dislike of disbelief-inducing thoughts is a sign of faith. The presence of intrusive thoughts due to masturbation emphasizes the obligation to stay away from it, and you must feel ashamed of Allah's awareness of you. Marriage is the best helper for you to do good, and you should hasten to it, as it falls under the three categories of people whom Allah has a right to aid: the warrior in the cause of Allah, the indentured servant who desires to pay off his freedom, and the one who marries seeking chastity.