The walls reflect light more than the front/back, so if you can go black there, it would be better.
A reminder: Flat paint. Not eggshell, not matte... FLAT paint always in a theater.
I'm not at all a fan of the cheap screen, and your projector deserves better, but 100" diagonal in a room 12' deep is about appropriate. You likely can't get much bigger with your depth.
I, personally, would recommend you spend some time looking at some home theater designs to see if there are any elements that you would like to introduce into your space.
I always recommend proper room lighting since I believe that most people completely blow room lighting...
Dark carpet, dark furniture for best results.