Taking the ceiling issue out for a moment, placing the 96s on the 15 foot wall would allow you to have 3-4 feet from the speakers to the side wall and 7-8 feet between the inside of each speaker. You could place them 5-6 feet (from the speaker back) to the wall behind them allowing proper space for these speakers to really open up. You could then dial in your listening distance and still have proper distance to the wall behind your listening position. This would be best with these speakers based on my set up experience.
I agree with your comment on the slope of your ceiling. The slope direction of your ceiling makes the best use of your room dimensions impossible.
Placing them on the long wall means you will have a wall directly behind your listening position ( 1 foot or less ) You should treat that wall with combo absorption/diffusion panels. This could work nicely. The room still works 🙂