1. Most Captains have a rule of thumb to estimate fuel required.
2. For airports where there are frequent stops, a handler is usually contracted to receive flight plan paperwork and give it to the crew. It is possible to transmit via ACARS, but it is not done often.
3. If you go to an airport often enough, you can guess. ATIS often has the information. The first approach frequency or the last center frequency will give the STAR and approach expectations.
4. Yes, via company radio or ACARS.
5. Any of the above.
6. Yes. HF is least used because of reliability issues.