Sorry you are in diagnosis hell. Autoimmune diseases often overlap. MCTD is a specific diagnosis and can include symptoms of sle lupus, scleroderma and polymyositis. It was first identified in 1972 and today often includes dermatomyositis and ra. Every case is unique. You can present with symptoms of one disease and then progress to the others. I presented with lupus and then polymyositis. Scleroderma did not affect me except for raynauds phenomenon.
To diagnose mctd your the anti rnp antibody has to show up. Otherwise, it will be called uctd (undifferentiated connective tissue disease). Based on the information you gave it seems like you have mctd.
Every case is unique and many I have seen are mild and controlled. Actually, mine was the worst of anyone I have ever spoken but was brought under control. Don't freak out. You are not new to autoimmune diseases so you should have a good understanding of them. They are not like other diseases. Just make sure you have an experienced rheumy treating you. I had an extreme case but it was finally brought under control with ivig and I have been in remission since Jan 2006.
Where do you live? Let me know what other specific questions you have.