What an awesome field to get into Sybille!
I agree with you. Methods of learning calligraphy can be used to help improve handwriting. So many excellent points already covered. Another I would add would be to teach them to self-critique themselves in a kind and patient manner; to learn to identify their poor habits (e.g., u and n looking the same, not closing the o, etc.) and work to correct them.
Another thing to keep in mind is why the person is coming to you for rehabilitation in the first place. The points you outlined definitely apply when the issue is mental in nature or from lack of knowledge. But if they are there because of physical inability, then all the studying of proportions and repeated practice will not help if the physical issue is not also addressed. And if there is a physical component, the type of physical issue will influence even that approach.
As for the comments about differences between handwriting and calligraphy, I have terrible handwriting...but I can also have excellent handwriting. I don't think it's just a matter of being unwilling. Handwriting is tricky because we write under all sorts of contexts and moods. How I write a reminder note to myself vs a birthday card to my niece or a thank you letter to someone will all look very different. Also, if I am angry, happy, sad, distracted, rushed, sick,....theses are all extra factors that will show up more easily in handwriting. With calligraphy, we train ourselves to suppress these things as well as our personal quirks in order to 'stay true' to the letterforms. The awesomeness of handwriting is that we have much more freedom in how it looks in the end....but often that freedom is taken too far to the point of illegibility. Also, intent matters. The end goal of calligraphy is typically not a grocery store list but rather a piece of art. Even in practice, being able to (eventually) make some form of art is usually the driving desire. Now, we could open up the old can of worms: is Spencerian really calligraphy? But even when writing Spencerian in modern times, the person is most often using it to make a desirable final product. With handwriting, desirable product isn't always the goal. Sometimes it's more important to get the thoughts down. If possible I try to use typing when those moments pop up now.
A final comment on neuroplasticity vs rehabilitation. My dissertation was on neuroplasticity; I cannot resist. I often hear the two terms used interchangeably, when mean different things. Neuroplasticity can be a part of rehabilitation. But rehabilitation can also involve other compensations or modifications to get around the problem. And
@jeanwilson, you so so right, it is very hard to convince people that the brain can change. It does.... on a daily basis, whether you want it to or not. This is why our habits, behaviours, and actions are so important. Our brains change for better or worse depending on our choices.