I'm sorry to hear that your application still hasn't been assessed. I've already received the outcome of my application, albeit unsuccessful, I had to push them to get my application through. It was processed by the faculty I applied so if you're applying for Arts, it would go through the Arts faculty.
At this stage, they should have processed nearly all, if not all of the applications. This week is O-Week so they should have let you know already. Knowing some administrative UoM staff, the best course of action I would recommend would to find some time and physically rock up to the Office of Admissions and ask them to personally resolve the issue right there. I can understand your frustration and I absolutely loathe being given the "cold shoulder".
We've spoken about this before and personally, I think this is pretty slack given that this is a mistake of their behalf, not yours and you are forced to wait. Unfortunately, I'm not sure if there are any official avenues for complaints for non-students but if you were a student and this was a problem with a particular faculty, I would have given their backsides a good kick.