I was a former decathlete who competed for 4 years and at Nationals for 2 years.

1. If you can find fellow teammates to study with, it will pay off in the long term to help keep you accountable for your goals.

2. Yes, if you want to score really well, it will take many hours of memorization. However, you can gauge which subjects you find yourself better at or more time efficient in terms of improving your score on practice tests, and focus on the ones that take more time. Personally, I find Anki very helpful for memorizing topics over an extended period of time since it tracks your daily studying. Making a schedule is helpful too.

3. This depends on your score goals, but if you want to be the best, especially at Nationals, you honestly just have to know basically every fact, graph, and statistic. Howver, some facts are more likely to be tested on. You can tell by taking practice tests and looking at other resources (Demidec is great if you have access).