Carfree: Your words: "I started this thread to ask for guidance precisely because I see it as a difficult problem. Even if no one has the secret formula for convincing elderly relatives to stop endangering themselves "
That is the point. There is no secret. Set things up so all the person's transportation needs are met. One person even gave you a useful checklist. Treat them like valuable humans and the details will work themselves out.
Don't want this to become personal so I'll bow out now.
Your post overlapped mine. Involuntary changes rapidly to when concerns are taken care of.