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.
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.
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.