I consider this handwriting business to be about half a clue.
In general, I believe people feel comfortable communicating with other people in writing, especially strangers, in predictable ways. I write in cursive, but I will print for other people because my cursive is atrocious. This can't be too controversial, we've all experienced this at some point. So, cursive was a part of the education of any Cooper suspect, but Cooper felt comfortable writing in cursive, for strangers, to commit a crime. And apparently it was "legible" (unlike mine). I don't believe this handwriting business will do anything other than represent one small part of eliminating marginal suspects. Barb Dayton, when she was writing for other people, wrote in a messy all caps (or most caps) print. She likely knew cursive, but I don't think she ever wrote in it, based on the documents included in the Ron and Pat's book. Thus, one "minus" on the checklist for Dayton. That's it, just half a clue to go with the other problems with Dayton as a suspect. In a lot of ways, I think the best we can do in this case is eliminate suspects, and this is one way to help do that.