As far as I understand it they will not really accepting bitcoin. They will use bitcoin (+ likely alts) so you can buy some StarbucksCoins. You can use those StarbucksCoins for coffee
No, no coins. They, and users, will have to open an account at Bakkt. Users load some coins into it, and pay at Starbucks with Bakkt app. The $ amount is debited from user's account.