1/2/3. If you made a mistake in your names order you can modify the sorting from the Joomla! Tags component (from the timetable, you'll have to modify the order each time you save the Timetable because column names are sorted by Id).
4. As Column names are managed by the Joomla! Tags component, you have to go there to manage them.
5. As you can see in my example, "Mercredi" should appears after "Mardi". You can modify them by clicking on their names
6. Change names and alias, correct spelling issues and save.
7. Words are well sorted.
Note: If you want to delete them, the procedure is the same as any Joomla! component: