At first I was thinking that, given the difficulty even placing sides/rears that 5.1.2 would be the clear answer. Looking at it some more though, I think that 7.1.2 or 7.1.4 would actually give you a better "bubble". You're rear surrounds will be mismatched, but if you run room correction/level matching it should be fine.
View attachment 79431
The "2"s are for atmos in a .2 setup and the "4"s are if you went with the .4
Again, not ideal but it can work ok without being "perfect" if you leverage software. And I do think that the flawed 7 channel bed will work give a better surround effect than the flawed 5 channel given your mounting location options.
The sub I would probably place under the TV on one side of the center. Unless you can find a sufficient smaller one that will fit within the 1' space beside the couch.