I am not a Down's person, and I haven't had strokes, and I still have trouble with poor peristalsis. I think it is a result of years of chronic inflammation. I have found that miralax at a low dose works well for me, but everyone's different. There are many people on this forum who suffer from similar issues.
Have you tried senna for a more direct stimulation of the bowel? I realize that it's not desirable to use a powerful laxative like that on a regular basis, but on some occasions perhaps it's reasonable. I also hope that as her inflammation heals she will not be having so many problems with her bowel function.