MCTD is a disease and I also have it. If lupus is complicated then mctd is much more so because it can present symptoms of SLE, polymyositis, and scleroderma and sometimes includes symptoms of other AI diseases. It is NOT the same as lupus although some patients do present mostly with those symptoms.
In my case lupus came first, then PM which was by far the worst. The only problem that might be associated with scleroderma for me is secondary raynauds and that has gotten better over the years. Treatment is pretty much the same as lupus but will vary depending on how the disease presents and progresses in an individual case. The marker that defines mctd is a positive lab for anti RNP antibodies in addition to the clinical diagnosis.
Overlapping connective tissue diseases are often difficult to diagnose and are highly variable but usually treatable.
Good luck and don't hesitate to ask lots of questions.