I see, good question. Well the report button was an afterthought, or more precisely, it was never designed to be a part of core script in the first place. years ago a user called Bloodrun designed a PM box mod with the layout, it looked nice and I took the idea to the core script. This is the page of the original PMBox mod, and as you can see the attachments dont exist anymore, since all attachments were lost as the forum was upgraded from MyBB to VBulletin.
http://mysidiaadoptables.com/forum/showthread.php?t=837
Since his mod's has lost all attachments, I had to simply re-create the PMBox based on the idea from his thread from a screenshot(which you should be able to see from the url above). The PM reporting feature was not in the core script yet, and and I didnt bother making it. At that time, it simply led to a non-existent link that shows 404 not found error page, it caused a lot of confusion back then.
Some time later, I decided to add this basic feature, just so that the button would work. However, it was of the least priority and I didnt want to spend much time on it, so it was done the way it currently is now. I guess this answers your question why the PM reporting feature works strangely, although it does work.
I like your idea for its improvement though, it looks really nice. Maybe I will take your idea for Mys v1.4.0's PM reporting system.