You can always flip the speakers - tweeters on the bottom.
You can buy the projector - my advice though would be to wait until your room is almost done - then buy the projector, test out on a temporary mount (boxes, chair, bookcase, etc.) to validate the projection mounting distance, and project some test movies on a temporary screen (use white bedsheet tacked to your false wall, to evaluate different screen sizes from your seating distance. That way you can dial in the size you like best (which may different significantly from any SMTPE, etc. guidelines). Then order and install the screen last (well, last except for any fabric panels you surround the screen with on the false wall).
That was the approach I took, I ended up with the front row 12' from a 136" wide screen (2.125 aspect ratio - so I get 149" diagonal 2.35:1 movies, 131" diagonal 16:9 movies), and to me its the perfect size. I watched movies for a couple weeks at various sizes on a temporary white muslin screen before deciding.