Yeah, you're right in what you say. 18 weeks isn't long at all really, and from the sounds of it you are fully functional with some annoyances. They will pass, just need a little patience, you're doing well, but i understand why you would think it would be over by now. All our experience with being ill programmes us to believe that most illnesses pass in a few weeks, and so should this. Unfortunately not.
With regards to the Remeron, as you said, you've already been advised to not taper it yet and I'd tend to agree. But it's your body, and your choice. As long as you realise things could get worse, as you could destabilise your CNS further and ultimately you'll be dealing with 2 WD's. That being, I have no experience with Remeron, and don't know if people generally get WD from it or not.
You'll get there, and it sounds like you are well on your way