5
You can change the default comment display mode to flat in administration/preferences/general settings.
This works for new members (anyone who signs up later on), but I don't think it will alter the setting for your existing members (at least, it didn't on my set up). If you don't have many you can change the setting for each one manually in admin/preferences/edit users.
I don't know how to do everyone at once. You could probably tell mySQL to do it for you but I'm still trying to get a handle on that.
