Mirror not working in game

You should send your package file instead.

The link you posted just leads to an error. Upload your package file to somewhere like Mediafire or Google Drive, copy the link, and paste it here. If you use Google Drive you'll have to allow sharing. Mauvemorn has a screenshot here that will help. 

Edit: Never mind. The link you shared was only partially...highlighted in blue...I guess you could say. I just had to copy and paste the full URL into a new tab. 

Just took a quick look and unfortunately, that's all I have time for right now. 

First, the mirror is way too high poly. It has more than 13,000 polygons. In contrast, the EA mirror you cloned has 46. You'll need to drastically lower the polycount. You can use the decimate modifier but be warned, it can wreck the topology. You can also hold Alt + Shift and select some edges, then X, then Dissolve Edges. You can use a combination of dissolving edges and the decimate modifier. It's your choice but ya gotta get that sucker reduced. 

I think the reason that the mirror isn't reflecting is because the back of the frame is clipping through the mirror portion. You can see this better if you select the frame and go into edit mode. Just select the mirror portion and move it a bit along the Y-axis. Press G to move, and Y to constrain your movements to the Y-axis. Hopefully, this will fix your problem.