Here's the placement guide for the wide speakers from Audyssey that might work better in your setup;
"The ideal location for the Wide speakers is along the side walls at 60° out from the center channel. To find the right spot, measure the distance from the center speaker to the left speaker. Then double it. That's where the Left Wide speaker should be placed. Repeat for the right side. The idea is to separate the Wides from the main left and right speakers so that their content is clearly coming from a different (wider) direction."
Well, that's not going to help. I was going to suggest something more radical, like moving the seating a couple meters forward.
If you blow across an empty Coke bottle, you can get that small chamber to resonate. Enlarge that space to your 540x282 cm room, and that chamber will still resonate (produce strong peaks & dips at certain frequencies). For your room width, those frequencies will be at 61Hz, 122Hz, 183Hz, 244Hz and 305Hz. You can eliminate the first one by placing your subwoofer at the middle of room width, either on the front wall and/or middle of the back wall (if you have a 2nd sub). The rest of those width resonances can be cancelled by spreading your front L/R speakers 188cm apart. This will give you smoother sound across your entire seating.
The problem with having your speakers only 188cm apart is that you could end up with a really pinched soundstage. The way to avoid that is by moving your seating forward and/or moving the speakers towards the seats. For example: you currently have a tiny 36° soundstage; moving 2.25m from the speaker plane would give you a 45° soundstage and moving 2m from the speakers would result in a 50° soundstage. You can see where I'm going with this: if you want anything close to the speaker angles that Audyssey recommends, your entire set-up would occupy only the front half of your room. That would give you much more consistent spacing between your fronts, wides, sides and rears. Currently you have your fronts far away from you and the rest of the speaker pairs clustered around your seating, with a big open gap in between.
Unfortunately, moving the cough to the front enough to get the right angle on the frontals is not an option! But, if I did, the distance between left and right surrounds and the wide would be the same. Both speakers on each side would move towards the front iqually.
My only hope is, as I described above, if the wides could be closer to the frontals becouse the tighter angle between them. If not, maybe I would use just the height channels...
Another question, even if everything works and I could install the wide speakers properlly, they will be small satelite speakers, is that a problem?