>"You're only doing half a job so we'll only pay you half your wages".
I can't see them halving your pay, especially becaus eteh minimum wage probably prevents this.
>"Can I be punished for not being able to do 3 people's jobs?"
It's not at this stage but i doubt they'd put it in those terms. If there's a chat you'll have the opportunity to discuss with them your concern about workloads and agree performance measures. Good luck.
After your break how long is your final stretch? It sounds as if you're entitled to a later break