Will a Muslim be held accountable for words he utters to himself, without speaking them aloud, when they are blasphemous towards the Divine, and uttered under the influence of severe obsessive-compulsive disorder, knowing that he did not intend their meaning? And can fasting for two consecutive months be an expiation for that?

No act of disbelief has emanated from you. Rather, what you have mentioned indicates the severity of the waswasah (whispers/obsessive thoughts) that has afflicted you. Strive against these waswas and endeavor to rid yourself of them. Your aversion to them is a sign of the sincerity of your faith. You are upon Islam and have not departed from it, and no expiations are incumbent upon you.