I too, have MS and am no stranger to the challenges that we face on a daily basis to do even the most essential things. Would it help you any to relocate your foot pedal so that the side of your knee pushes the pedal instead? Some of us MS'ers even put the pedal on our table and use a hand to press it.
Another possible option that is only available on some sewing machines is to use the "Start" and "Stop" button which makes the machine sew and stop without the use of a foot pedal. I tried this out, and decided it was pretty handy for the longer seams, but not so much for the shorter ones or where my precision is required.
Heat affects most of us with MS to some degree or another, to the point of making some activities very difficult to do due to loss of coordination, problems with vision, even mental acuity. If you are bothered by the heat, you might try any of a variety of things to make your sewing time more enjoyable. I use a fan in my room when I'm too warm. I also keep ice water or iced tea nearby and drink often to help keep my core temp as low as possible.
There are also those "cool collars" and even the vests that use special ice packs in them that really work to keep your core body temp down. Sometimes, I have to wait to sew til the evening when it is much cooler.
You probably know too, that having an iron on heats up the sewing space significantly, as do many household light fixtures. I use my Ott-Lites to help cut down on the heat and to give me better light to see by.
Anything you can do to minimize how the heat affects you should help. Hope you've found some of these suggestions helpful.