Would diffusion help in this space

Wow, you've been playing with this for a while!

I haven't read all of the posts involving potential solutions so forgive me if I miss the mark by a mile. If your latest photos reflect your present speaker set up, and I have read your comments correctly, your speakers are seven feet apart and your listening chair is only 4 ft back. I would think that first reflection points on the side walls are not an issue and neither diffusion nor deadening wall treatments would help you much, if at all.

You didn't ask, but when I look at your photos, what stikes me initially as being a potential problem which will screw up imaging is listening to the speakers from 4ft away. Something closer to an equallateral triangle would be much better. Move your chair back to 7 ft or move the speakers closer together. Personally I'd try to get the listening position further back if you can. Perhaps a combo of both. In theory at least, you would get a better centerfill and solidify pin point imaging (which is what is necessary for great depth of image). In theory!

Good luck.

FWIW.

My answer may have been inappropriate due to my misunderstanding. IMHO, great depth of image (pin point imaging) can exist only when you have achieved optimum resolution in equipment, room and set up. (My biggest improvement in depth of image came with boxes with SOTA tweeters).

Diffusion could help your sound if placed on the wall behind your chair. Although I doubt it, it might even help on the side walls as well. So could some deadening materiel. The frequencies that bring depth of image to life are the high frequencies so you can (sorta) replicate the effects of using expensive materiels by simply hanging some heavy materiels on the wall(s) temporarily and see what happens. If you get no change at all in depth of image then you can figure you are barking up the wrong tree.

FWIW. It is hard to fine tune a system over the internet. :-)