Most of these questions are entirely speculation, different people believe different things. I'd recommend trying out your ideas on 1 account at a time until you find something that works. Biggest mistake I think people make is jumping into running 10+ accounts at once and then not earning your money back when it doesn't work due to lack of testing
In my opinion:
1. The API is easy to use + good tutorials available for CLI
2. You can buy pre-made accounts / run an account creator via your proxies
3. Up to you, I wouldn't bother
4. I'm not convinced they make that much of a difference, but a rule of thumb is datacenter/corporate proxies for suicide farms, if running something a bit more then get a residential proxy
5. You can buy proxies on a shorter term (1-2 weeks) if you're worried about flags