1) re arthritic fingers: I've seen fiddlers in their early 80s and I think what keeps their fingers nimble is the fact that they've kept playing. "Hudi McMenamin" recommends some exercises such as putting an elastic band around your hand and stretching/spanning your fingers against it, also massaging your hands and manipulating the joints of each finger.
2) I know a couple of guitarists who are missing some fingers. Of course they adjust their styles accordingly
3) I saw a photo in a book once of a fiddler (Australian, I think) who'd had his left hand amputated. That's normally the fingering hand, but he relearned the instrument, fingering with his right hand and holding the bow in his hook. Incidentally, someone told me that it's bad for the structure of the violin to reverse the strings (the way left-handed guitar players apart from Elizabeth Cotton generally do); only the chin rest should be moved when switching playing sides.
4) I play despite a disability - insufficient musicianship!