Wow, well here we go:
1. You can use the same layout but the code will need managing better so you don't end up with crazy unmanageable event sheets, I advise you use a new layout if its a small project.
2. The the new layout loads, the old layout will stop and destroy all objects. If you want it to not be destroyed, all the items inside it must be "global" (see object properties). If you want their code to still run, their code must be in a separate event sheet which is also loaded into the new layout.
3. This is the correct way. If you want the game state to persist there are a lot of options, try looking at the "Save" actions and the "Load" actions also the "Global" setting. A lot of solutions here.
Edit: As the below poster recommends Persist behaviour below I agree is better solution. Ignore statement 3.
Hope this helps, let me know if you have any questions.