So sorry your daughter has mctd. Like all auto immune diseases presentation, progression and response to treatment can vary widely from one patient to another. No one, not even your doctor can tell you how her case will progress or respond to treatment. Plaquinel is a relatively mild drug given to mctd and lupus patients with mild cases to see if it can control the immune system. It might have some impact immediately but usually takes many months to reach its' full impact if it works at all. More typcially, prednisone is given as the first drug and then a prednisone sparing drug started and pred tapered slowly due to it potential side effects from long term use.
MCTD cannot be cured and by definition can exhibit symptoms of lupus SLE, scleroderma and polymyositis. A given case can have symptoms from any or none of the 3 and to widely varying degrees. I was diagnosed with first lupus then mctd 6 years ago and had an extreme case with severe symptoms from lupus and later PM. Some cases of mctd might also include other auto immune diseases. It is a complex and highly variable disease that can be almost unnoticeable or very severe.
The good news is that most cases are treatable but some cases are more severe and resistant than others. As a result some patients can go through a process of trial and error to find the right med or meds to control the immune system and that can be frustrating. Normally rheumatologists treat this type of AI disease but most are not experienced. If you are seeing an internist or GP I would suggest going to a rheumatologist in a large teaching hospital. If you look for doctors experienced treating lupus you will have more success.
My case was extreme but my docs were able to "tranquilize" my immune system and stop the attack on my body. Despite the severity of my case I have not had a flare since the initial one. Most patients have ups and downs with periods of no symptoms and then flares.
By the way, there is a nurse in my lupus support group who got mctd while in nursing school and was able to finish. There are 3 others with lupus who are also nurses although 2 of them had to stop working to get their problems under control. MCTD or lupus alone can have cognitive effects and make school difficult to complete.
Here are the sites that have very good basic info on mctd but none will help you to predict your daughters.
I wish I could give you more definitive answers but that is impossible. It is important to treat the disease aggressively in the early stages and if she experiences symptoms of PM physical therapy is important to maintain and restrengthen muscles.