Stain vs paint maintenance
To answer part 2 of your question.

You should probably reapply the stain a little earlier than the full 5 years - it could be starting to show signs of wear before then and you don't want to let the weather in.

If you are reapplying a stain then cleaning and lightly sanding the door should be preparation enough. If you want to apply a lighter stain then you will have to remove more of the old finish first.

Even if the stain does peel (which is unlikely) then sanding should be all you need to do. Be careful though to make sure that you sand the door evenly. If you take more off in one area than the rest it will show through.