What is the difference between a thought (حديث النفس) and an intention (النية), especially since the Maliki school of thought holds that divorce occurs by mere intention, and what should a person suffering from obsessive thoughts do if he cannot avoid them?

Divorce does not occur through self-talk or mere intention, even according to the Malikis. Sheikh Ulaish clarified that a mere intention does not bring about divorce by consensus, and what is meant is that a person articulates the word of divorce to himself and brings it to mind without uttering it. There is no effect of waswasah (whisperings/obsessive thoughts) or thinking about divorce in one's mind. One must disregard waswasah, and it is permissible for someone afflicted by waswasah to adopt the easiest rulings to alleviate hardship.