Agreed... 5) is minor and just the result of saying "We're not going to bother trying to track if a file goes from having unsaved changes to not as a result of undo/redo - once a file gets marked as having unsaved changes, the only way to 'clear' that is to save the file again."
That option is much better than getting the undo/redo vs unsaved changes tracking wrong and allowing unsaved changes to be easily lost, like notepad does. :-P
0) Set "When Notepad starts" option to "Start new session and discard unsaved changes"
1) New file
2) Type "abc" - note the "X" to close the file changes to a "O" (pretend that's a circle) to indicate unsaved changes
3) Save file - "O" changes back to "X"
4) Type "def" - "X" changes back to "O" because "def" is not on the file on disk
5) Undo, "def" disappears, "O" incorrectly remains...
6) Redo, "def" reappears, "O" incorrectly changes back to "X"
7) Close file. No prompt to save unsaved "def". Reopen file. "def" is gone