I was concerned about the same thing. The couch shown in there is just one I grabbed from Google's catalog. But for reference, our couch we own is about 94", so that would only leave about 17 inches per side. I am guessing that is why in your one suggestion you moved the AV cabinet to the back corner, so you could walk around with more space.
For the height, where the post are it is about 93", before the ceiling. Then just above that in the picture is the duct work and sewer drain. I could bump it up a little once I get out side of that, maybe another 6" or so. I do have this other nice, sarcastic, air vent that sticks down and raps around the floor joist. But that is still above the 93" height.
The post to the steps is about 2 feet 9 inches, give or take.
Does anyone know the recommended height from the ceiling to the side and rear speakers? I know they say at least 2 feet above your ear. Also, how deep does a AV cabinet need to be?