How to reserve seat before valid date

I plan to travel from Sapporo to Hakodate on my second day but I will start using my JR Hokkaido pass on this second day, too.

1. Can I reserve seat to Hakodate before the first date of my JR pass?
If yes, how?

2. Can I exchange my exchange order into actual rail pass but not start using the pass yet? For example, I arrive Japan on day 1 and exchange my exchange order into actual pass on this day, but the first valid date of JR rail pass will be on day 3.

3. Can I make the advance seat booking for the rest of my trips such as for day 4, 5? How many days are allow to book in advance?

Thank you for your helps.

1. Can I reserve seat to Hakodate before the first date of my JR pass?

Yes, you can start making reservations with the pass once you have it in hand (even if it isn't' within its active period yet). Of course you would only get free reservations for trips that fall within the active period of the pass.

2. Can I exchange my exchange order into actual rail pass but not start using the pass yet? For example, I arrive Japan on day 1 and exchange my exchange order into actual pass on this day, but the first valid date of JR rail pass will be on day 3.

Yes

3. Can I make the advance seat booking for the rest of my trips such as for day 4, 5? How many days are allow to book in advance?

Yes, see my answer above

http://www.japan-guide.com/e/e2361.html

All your answers are at the bottom of this page.

Thank you.