How bad is it to place 5.1 rears from behind rather than on the sides?

Thank you all for sharing your valuable experience. It sounds to me that weighing out the different circumstances of my house, monopole sides will work best for me. Though before mounting them on the wall, I’ll first give them a try both on the sides as well as on the back.  

There are some surround speakers which have two pairs or drivers and are switchable between a dipole or bipole configuration.  If you really wanted to see for yourself, you could try the "dipole" sound and compare with the "bipole" (which essentially give you the same effect as monopole).

You are so close to a nice 7.1 set up with all 7 speakers on stands. All you need do is pull the couch forward and little, enough for speakers to be as they are in in your first figure, behind you. a bit wider than your L/R pair. 5.1 to 7.1 is addtional speakers on the sides, toward the back of the room maybe 3/4 of the way. You could then add 2 or 4 speakers overheard in the ceiling and you’d have ATMOS! Small rooms actually are better for this if you dont over power them with big speakers. Nice small speakers like Genelecs or soemthing like that would be ideal.

Erik is right, dipoles are a thing being "sold" to consumers; nothing is mixed that way or built that way in the studio. I would not use them as nothing would sound its best that way. Especially when standard speakers work fine and are easiy to move around until you get a good "blend" between front, sides and rears (where you dont hear a big dip in level between speakers from a source that moves across all of them).
Brad