Are the expenses for food, drink, and restrooms, in addition to money missing from customers, the responsibility of the worker or the employer? And what is the solution for attaining devoutness in prayer, despite performing all its pillars, when the worshiper's mind is distracted outside of prayer during prayer times?

You must refer to your agreement with the employer regarding eating and drinking during work. What he has permitted is permissible, and what he has not permitted, you are not allowed to do at the expense of the business; rather, you must pay for it from your own money. As for a customer who does not pay you the full price, you are liable for the difference to the employer, because you are not permitted to sell for less than the set price. And devotion (khushu') in prayer is one of its most important aspects, and if it is missed, the prayer is not invalidated. You can review the fatwas related to increasing devotion.