Need help with room problems

My specific issue is that my "sweet spot" is very small and quite dramatic. My "system" link will show you some visual details but here's more info.

My room is about 16'x 12'x 8'(height). System is on the long wall. Speakers about 23" from rear wall, 6'3" apart and my head about 8' from them. No toe-in as recommended by the manufacturer. Room is flanked on the left by a large window starting 3' from the floor and is centered measuring 4'h x 7'w. It is covered with curtains, though not heavy ones. The right is open to the dining area, and a half wall that leads into a hallway. Two large bookcases are at the first reflection point to try and minimize the chaos of the hallway.

If I move my head more than about 6" the placement of the instruments change quite drastically. The effect seems to be with the right speaker. Sometimes the sound is as if from the speaker and in the sweet spot, the speaker "disappears".

I looking for suggestions as to what to do to deal with these problems.

Thinking of trying the system on the short wall with the window behind it but I am concerned about the bass response due to the ports. Also wondering if the open dining area and the opening to the hallway will send the sound off into oblivion.

Thinking of diy roomlenses. It is the living room and unfortunately the only possible room. Also we rent so I can't alter anything but the furniture.

Thanks.