You have to buy an Orca card with cash, then pay the fare with the Orca card. It's indirect, and any remaining balance isn't in your pocket. The point is: you can't pay cash at boarding.
You don't have to buy an ORCA card. The ticket machines dispense both paper tickets and ORCA cards. The ORCA cards cost $5, so most tourists opt for paper tickets instead.
No, you don't have to buy an ORCA card. You can buy a paper ticket for the exact fare at a ticket vending machine, which are available at every Link station.