#1. This lens is brand new to me, I have not learned it just yet, One of the I could have done to make it better is stepped it down one stop and maybe 2, but I like to blur the background as much as I can. It takes a while to learn what a lens can and cannot do in any giving situation.
#2 The darken background is not from the original shot, there was a vignette added to the image in post processing.
#3 Background are one of the most important part of a image, I always try to align a subject to a nice background.
#4 backgrounds can easily be changed, for example using foam Board, it's available in many colors, one of my favorites is gluing black velvet on to a foam board, this way you don't have to dig it up and can still use a off camera flash or soft box like you would in a studio.
#5 with today's software one can easily replace the background with a digital background, The best software to do this is On1 RAW.
#6. by using a layer mask one can also easily brighten or darken any part of the image and not effect the whole image at all.
I sure hope I answered you questions if not please let me know.