I like the corner setup, but only you know how the room will be used. Chatting would dictate more face-to-face, while media viewing would be different.
Having built a 'music room' for my one seat and audio system, I can offer a little. The is acoustic room treatment for internal sounds management and other for sound isolation, which is what insulation and all does. I built a 'sonic Thermos'; a wall within a wall, each almost totally isolated from the other. The ceiling 'floats' on Z-strip and is three layers of 5/8" drywall, glued together. All edges are foam-stuffed to reduce mechanical sound transfer.
My fine electronics (Cello/Levinson) are on their own circuits with hospital grade outlets and #8 wire from the CR box. A power conditioner holds the voltage steady and filters the minimal line noise.
No room dimension is a multiple of another. Were the room longer, I would get stronger bass inside, rather than what I hear outside the door. The room is not deadened with heavy acoustic drapings and I don't feel I need multiple vertical strips or special refledtors/traps to get fantastic audio. Think of sound waves bouncing about with different wavelengths hitting differing places. Your 'corner design' strikes me as better from this standpoint, versus a boxy room.
The sound will come primarily from the speakers and I wonder if there is really much practical difference between 5 and 7 channels, plus subs. It depends on your software (sources), but 99% of my material has very little driving the 5 effect speakers I run in my (other) family room/video theater. Very occasionally, I hear some thumping from the rear sub-woofer and only slightly more often, do I notice side or rear activity from the effect speakers (as it should be).
There is inherently more compromise when building a multi-purpose room. The .W.A.F. (wife acceptance factor) is huge. There are competing focal points, but an exposed large screen is very compelling.
I am working on a large-plasma-over-fireplace family room. I tire of the harping about such an install as with proper shielding/spacing heat effects on the screen is reported as minimal, but those who have measured it. Too-high mounting should e carefully considered and managed, but this is a 'compromised' room. Our seating allows reclined viewing and enough distance to avoid neck/eye strain.
Visit and observe the wall treatments and shapes of aesthetically pleasing and sonically superb rooms, private and public. Then work a design that pleases and serves your special group of consumers.