You need to call the police and file a case against them in civil court. Don't wait any longer and get it done. That's BS they pulled that and if they were in your face like that, then it's time for a lawsuit. Dude, I'm 18 and I don't give a damn who it is, they are supposed to speak to you as if you were an adult. If not, then it's time to call the fuzz. What these old pecker woods don't realize is that not all of us young guys are complete idiots like a lot of them think.
Now I can understand a possible complication with running a 2.5 inch exhaust through a monte. I wanted the same thing in my malibu (exact same undercarriage). Problem is that you can only use 2.25 inch exhaust to go over the axle tubes cause the 2.5 simply won't clear. That's why I stood there the entire time when I had my exhaust done and criticized every little thing I saw wrong. So what they had to do to my malibu was put 2.5 from the headers to the flowmasters, then 2.25 over the tubes and out right behind the tires. But as far as they treated you, if I was there with you brother, I'd walk in there with you and raise hell. Take them to court and get the law involved.
I only paid 240 for my exhaust, and if your paying that much, I'll tell you right now that exhaust better be god damn perfect.