I don’t often re-read books, but when I do, it’s because it’s been so long I’ve forgotten about what happened. You will be so surprised to see how different you feel about things in the books after you’ve had more life experience. It’s fun to revisit some books to view them through a different lens.
I don’t. If you literally didn’t finish, it is not completed. You can track it if you like, but it should be tracked as DNF, and it shouldn’t count as a book read.