Nah, you won't get fired. It's also partially the shifts fault for not checking who you're covering for.

Might have a talk with your manager, shouldn't be a big deal (although kinda pricy for labour).

Hey so long as you get paid for both days. Starbucks can afford it. DONT LET THEM NOT PAY YOU!

Did no one notice that you weren’t supposed to be there?

LMAO was it just a training shift? If so, it probably doesnt matter anyway. Or were you working the floor? But really if no one else noticed, i doubt its a really big deal. Talk to your SM and get it sorted out asap, probably because they wont like if you pushed yourself into OT, but I think youll be fine hahahaha