The only drainage I’ve ever had was from treatable ear infections or fungus. I wear 2 hearing aids. I see you asked about leaking of spinal fluid and that you are doing research. Has the ENT mentioned if your eardrum is ruptured or perforated? Can you use cotton in your ear at night to absorb some of the moisture? Does it leak during the day when you are upright? I think 7 months is a long time to go with no answers although I know drainage of this kind sometimes takes a year or more to resolve.
Since he doesn’t know what is causing this I am wondering if you should seek help elsewhere. How long has the ENT been practicing and how often has he seen this problem. I am sure he will understand your frustration and need for an answer.
Do you have any pain or do you wear a hearing aid or use ear buds at all? And do you feel any pressure before the drainage starts?