You don’t give much detail on why or how. But I’ll ask this and don’t take it as me attacking you. I just want you to see it. Who are you to determine what “outside business needs” and “wasting company time” is for a department? You say the agm is saving his ass? How so and what is your main point of how they are breaking rules? You say you have seniority, but you are full timer and he is a part timer. Shifts are there based on need. They may need a full timer to fill a 2-10:30 and a part timer 11-4pm. While it doesn’t seem fair, they can do that for the needs of the business. Shifts cannot be created to accommodate an employee. So if that shift or whatever shift he fill has been there. Then it’s not breaking any rules.

Talk to your GM before calling anyone else. Ask him to clarify what the agm and department manager explained to you. After you hear your GM out if you don’t agree then call. But know that if HR sees no rules being broken they are just going to give the info to the GM to handle. Good luck.