1. You will need to do a "factory reset" from the settings menu, but since I have not performed one, I cannot say if the screen layout will be the same as the original, but all of the apps will be restored to factory state.
2. The $25 credit promotion has expired
3. If you setup the tablet with the Google account under which the apps were purchased, they will be available from Google Play and you can setup multiple Google accounts , just remember the tablet will have access to your Gmail as well.
4. If you are at version. 4.1.2, after the factory reset you will remain at 4.1.2.