Hi. Dont Be anxious. MRI scans are very safe compared to CT-scans and although it may be intimidating, it is really nothing to be scared of.
The reason why they ask you to stay still is because the MRI takes a much longer time to 'capture' the image as compared to CT (which is almost immediate). A simple comparison who be to think how they used to take photos back in the 19th century photos where a person had to stay still for like 1-2 minutes.
In my experience, it is very common for patients to move around a bit. It is impossible for anyone to stay absolutely still for so long! Thus, small/ unconscious movements may mess up the image a bit, but not significantly to cause an issue. This is normal.
Many, many times the patient moves too much and the image gets noticeably distorted, so they usually also have 'do-overs'.
Regarding your first question, I advice you to ask your doctor (I am not sure myself, its better to get an expert opinion).
Regarding your second question, I would say no. Firstly, MRIs take too long, they wouldn't put a person who is not compliant in one and waste time. Even with restraints, if the patient struggles, it will mess up the image. I have never seen restraints on an MRI machine, but have on some CT machines.
Also, if they strap you up and restrain you, it would make you more anxious, which wouldn't help.
Regarding your 3rd question, that is correct. The minimum time is about 20 min. As discussed above, if they dont get a good image because a person is moving around, they they will 'do-over' which will add onto the time.
The key is to relax, learn a bit about the procedures during an MRI scan before hand so that you know what to expect (posting this question is a good start) and dont be intimidated by the command "dont move!!", what they really mean is to try and stay still as much as you can.
Hope I was of assistance.