As an exercise physiologist (gerontology) I work very closely with physical therapists. I get their patients when the insurance runs out. I work in an independent living facility with healthy, smart 65-93 year olds. My advice would be don't start doing any machine quilting until you have some physical therapy under your belt. Your first priority should be doing the physical therapy and not re-injuring the rotator cuff muscle. I cannot emphasize enough that you should do any home exercises the physical therapists have given you. This should be first priority. The people who recover from these injuries are the ones who work very hard in physical therapy and do their homework and don't do anything they're told not to outside of physical therapy. Talk to the physical therapist about starting to sew. Make sure they understand exactly what quilting involves as most of them don't really know much about sewing or quilting. Let them know this is a priority for you. If you're in a lot of pain as you sew, that's a sign to stop. It may make sense to do some of the physical therapy exercises before you start sewing so you're 'warmed' up.
It also may make sense to start with hand sewing. Perhaps a crazy quilt?
When my 92 year old mother was injured she had physical/occupational therapy. One of us was there with her the first 6 weeks. She had 90 minutes a day. She had to do 1 hour and then take a nap and come back for the rest. 8 weeks later she went from skilled nursing back to independent living, walking a mile a day. It was hard work but its doable. It's tough having an exercise physiologist for a daughter. Every day I ask her what exercise have you done mom?
I wish I could make you better today.