Two dipoles in the back where you described them would probably help to create spaciousness, whereas what you probably experience now with your surrounds is sound which is obviously coming from one of the speakers, rather than a sense of envelopment in a larger room. But being so close to the sofa, they may be too close. You could try mounting them higher so you have more of a sense of "I'm in the environment of the movie", rather than "there's a speaker behind my head".
For front height, I'd personally go with the larger Studio 20. I'm of the mindset that when things get loud in a movie, I'd rather spend more and have more headroom rather than spend less and possibly run out of headroom. You have huge mains and a huge center. You'll want to be sure the height channels can keep up. Looking at the 10s vs 20s, the 20s are 1db more sensitive, and can handle 20 additional watts. Judging from your room pictures, it looks like your ears will be about 4 meters from your front height channels. Because of this, there will be 12db of loss due to your distance from the speakers. So technically, neither choice would be able to give you 105db at the listening position if you decided to turn the volume up to reference 0. Using the 20s, the loudest you could go is -7db from reference, and with the 10s, -8db. If you never listen with the volume higher than -10, which is about where most people listen, then either speaker will work. But the 20s will have 3db of headroom wereas the 10s will have just 2db.