1. N-400 petition belongs to LPR, not US citizen spouse. To avoid issues, immigrant should have their own account.
2. You can file by paper if you wish. You don't have to file online. If you file online, you don't need to submit anything by paper.
3. Depends on you. If you're a quick learner you can wait. But it's never a bad idea to prepare early. Official learning material is here: https://www.uscis.gov/citizenship/find-study-materials-and-resources/study-for-the-test
4. The best way is when I-751 is decided before N-400 interview. If not, combo interview is the next best. N-400 cannot be decided before I-751.
5. It's fairly simple. Find all of your travel dates before filling application. Same goes with marital evidence and addresses for the past 3 years.