Must one who suffers from obsessive-compulsive disorder repeat the Maghrib prayer if he doubts having performed it, then performs it again, and thereafter prays Isha as a delayed combined prayer? And is it permissible to combine two prayers if there is a long interval between them?

Continue to strive against yourself to rid yourself of your obsessive thoughts. Your prayer is valid, and nothing is required of you, whether you prayed Maghrib once or twice. Nor is continuity required between two prayers combined in a delayed manner (jam' ta'khir). Be careful not to delay the prayer beyond its prescribed time. If you fear the time will expire due to obsessive thoughts, then intend to combine it with the next prayer in a delayed manner. Illness is an excuse that permits combining prayers according to many scholars.