I could be wrong, but this is what I was told about riding jackets and how they should fit.
The jacket should be snug but not tight. When you are holding the handle bars of the bike, the sleeves should be at the base of your palms (thus longer when standing).
As far as your air vent problem, I would agree that it is something that you probably would just have to get used to. I have a leather Joe Rocket that I have a similar issue with, however it is a little different. I wear an HJC CL-15 with a Chatterbox on the left side. Whenever I look to my left, the Chatterbox scrapes on top of the vent. It was pretty annoying the first few times, but I got over it pretty quick like.
As far as mirror use, I only use the mirrors to briefly glance at what is behind me. Whenever I am changing lanes or making a turn, I make sure to actually turn my head and look as far behind me as possible. This is much easier when I am switching lanes to the left as I can let go of the left bar briefly to almost turn completely around tp see what is behind...not as easy when changing lanes to the right, although I have done it.
To me, it sounds like your jacket is a little bit to small. The next size up would probably be better for you. Really though, I think that it comes down to what you are the most comfortable in. I personally like Joe Rocket jackets, but that is just me (I own 3).