Trust them or don't trust them, your choice. But the law does not obligate the employer to let you work overtime, does not require that you be allowed to work as a cashier and does not have any obligation to let you defend yourself. You may or may not have these rights through the union, but you don't have them by law. The employer is doing nothing illegal so there's no law you can invoke that will require the employer to do what you want them to. You can do nothing and have nothing change, or you can go through the union and maybe have something change. Up to you.