Do not mess up with embedded objects in Quiz slides. I suspect you just deleted the Back button, which is an embedded object.
To have them disappear from all Question slides, you have to go into Quiz Preferences, Settings and uncheck 'Allow Backward movement', then confirm deleting all Back buttons.
I don't know what you did to the duration, give some more details? Default duration is 3 seconds with a pausing point at 1.5secs. Maybe also due to 'messing up'?