Definitely, Water Fasting wouldn't be a good idea for you since you're already underweight. MH, on his website, has a book called the "Too Thin" book that you might want to read.
If you do a liver flush, I believe in preparing for at least 5 days beforehand by taking herbs so as to make sure that the bowel is moving, along with anti- parasite herbs.
As far as chelating goes, which is mentioned somewhere in this thread, I had a Naturopath put me on DMSO and I deeply regret it. I consider it to be a drug with harmful side effects that just puts more strain on the liver and the body as a whole.