MCTD is a separate disease classification. By definition it is lupus, scleroderma and PM but you might or might not have symptoms of all those. If you exhibit symptoms of lupus you do have lupus...if you have symptoms of PM you have that.... MCTD is an overlap disease meaning it overlaps several diseases which is quite commone with autoimmune diseases. It would be correct to say you have MCTD or lupus or PM or scleroderma IF you have symptoms of those. Read the links I provided and it should help you understand things better. The nature of AI diseases is such that whatever disrupts your immune system to produce X symptoms can also produce Y symptoms. MCTD is like that and very unpredictable but treatment is similar to lupus or pm. AI diseases are not like others where a specific pathogen causes a specific disease which has a defined pattern.
Both lupus and PM can cause muscle weakness but PM has a very distinctive pattern of weakness and an experienced doc should be able to determine that. In addition, those with active PM have elevated cpk numbers and that should be tested. PT/moderate exercise is important even if it is only range of motion, stretching and light resistance. Othewise you will be on a path to significant dysfunction.