It's actually the limitation that you have to put the viewing panel / main speakers on a long wall. I always recommend that people use the "short wall" for placing the tv / screen / main speakers because it usually works out better acoustically in this fashion. But this isn't always doable in certain rooms (like yours).
If you have to use the long wall for the main screen (like your room), then the subwoofer would be best in a corner for sure. In my opinion, the main left/right speakers should be relatively close to the center channel. I would say you could get away with putting the main speakers probably about 25% away from the corner. The left/center/right should be close enough where they present a more "cohesive" sound. You could always try them in the corners if you want, but you should definitely put some acoustic absorption panels within the corners to prevent standing waves in the midrange area. In the corners of such a wide room, I would worry that you won't be able to identify direction of sound sources between the front left/right and the surround left/right, since they are so widely placed.
For surrounds, technically the left/right surrounds should be at the same height as your ears. However, this is not always possible and can become difficult in a small room where you have one person that is closer to a surround speaker (i.e. if you're on the left, the left speaker is 2 feet away, but it's 5 feet away from the person on the right). So you have to make some decisions and compromises on placement so that surround speakers are somewhat equalized between all audience members.
I have a small room (at only 11 feet wide) and my surround speakers are on the side walls about 1.5 feet above listening. It works out okay, but there is still a little imbalance even with 2 people on my loveseat.
In your room, I would consider these positions:
1. Bookshelves mounted on stands and directed towards the listeners. (your idea).
2. Mounted on the back ceiling about 25% into the room angled down towards the listeners.
3. Mounted on the side walls maybe about 5% into the room at listener height (as long as there are no doors blocking). The sound would reflect off the back wall for the listeners, which is find for surrounds.
4. Mounted on the back/top of the side walls angled towards the listeners.
Options 2/3 will have the surrounds placed far away from the listeners, which should decrease the imbalance where a listener may hear the left surround more than the right surround.
I actually like your idea of putting the surrounds on stands. This allows you to adjust the position to where you think it sounds best.
