I have tried 21.9 version and I was pleased to experience that Comment.Note can be still set if it is a threaded comment despite of not having this feature in the 21.8.7 fix. It is an important feature to us and we really appreciate still having it.
Also, I still managed to set threaded comment author properties only once in Workbook.Worksheets.ThreadedCommentAuthors to update every instance of the same author which is a perfect way of working for us currently, but I realized your enhancements regarding setting threaded comment authors as well.
The information you gave about not to change Comment.Author when it is a threaded comment in order to prevent threaded comment structure from breaking was also really useful.
All in all, we are satisfied with this latest version of Aspose.Cells and thank you for your sustained assistance and cooperation in this topic!