I'm sorry you haven't received a response yet. It seems this board isn't very alive on the weekends. Feel free to submit your questions to the Lyme forum as well. People are very knowledgeable there, and that board is extremely active.
Regarding your question of ruling MS in or out via test, there really isn't one. MS is a diagnosis of exclusion. Depending on pattern and location, brain lesions can be from many things. You are right in that Lyme and MS lesions can appear the same.
I am not a professional, but if I were you, I would approach the Lyme diagnosis before you settle with the thought of having something that is incurable (MS). You are very lucky to have had a positive test. Many people with Lyme go months, years, and some NEVER get a positive test. Most are diagnosed clinically.
As far as your concern for the medications, that is entirely your call. Without a positive test, I decided to try treating Lyme first. I will say that my neurological symptoms did intensify, but I think you really have to talk to your doctor and weigh out your specific situation, risks and benefits. I took Valium to help when things got to be a bit too much for me. I'm still unsure of whether or not I have Lyme, MS, or something else. I figured I had little to lose and possibly a lot to gain by treating possible Lyme. I would encourage you to look at it in that way also. I don't know how bad your situation is, but mine was to the point of "I'll try anything."
Good Luck, and let us know what you decide and how it goes. Again, see what the Lyme folks have to say. You may find more people there in your situation. Both boards will offer support and advice.