I have a couple links for you, but i don't know they will help.
http://the-digital-reader.com/2015/0...d-export-them/
and
http://the-digital-reader.com/2017/0...es-highlights/
My concern is that I don't know how you loaded the ebooks onto the Kindle.If you loaded them over USB then the highlights only exit on the Kindle and nowhere else.