Niv, welcome. As Nick and you yourself point out, the in-ceiling "surround" speakers are far from ideal, firing almost straight down at you as shown in the diagram. As was mentioned, several of our members who didn't have wall space or want to use stands, use brackets to suspend QSs a short distance below the ceiling.
Apparently you already have the ceiling speakers, and if they're going to be used the first point is that the side surrounds should be farther back and spread as widely as possible(even if the ceiling makes them uneven in height), so as to give a little more side rather than overhead effect.
The back surrounds may be of very little benefit if placed as shown(and if Nick's suggestion of mounting them on the wall behind can't be used). You might consider instead using them as front "height" speakers which the 4311 provides for, above the main speakers. In that case the side surrounds could be mounted a couple feet farther back than the couch, in the absence of the back surrounds.
Enjoy the music, not the equipment.