In-wall installation in wall that has no rear drywall

Well been studying up on the finer points all summer. I have my room layout graphed out, I have mock up paper cutouts of speakers and tv hanging on the wall, I have located all the wall studs, and thought through all the wiring routes.

ROOM / LAYOUT
Room is vaulted ceiling bonus room, F & R walls are 15 ft long, and 13" apart. Couch needs to be up against back wall. Upper part of ceiling is 9 ft. As mentioned, the Front and rear walls have attics behind them and are only dry-walled on the inside face.

I will locate the 55" TV about 34" from the floor so that seated eye level is about 25% up from the bottom of the screen, 75% from the top.
Center channel will go directly below, maybe 3-4" space between the two. Both are centered on/between two studs, which are almost middle of the wall.

L & R will be 2 stud cavities away from TV; one empty cavity between them. Ideally I would separate them a bit more, but since the seated distance from the TV is less than 13 feet, this should be good, and keeps the L & R within ~20 degrees off center from viewer. Also keeps them away from an attic door on the left and the computer desk on the right.
They will be slightly higher than ear level, however I am going with speakers that have aimable mid/tweet components with 18 deg of movement. Should be able to dial them right in and down to my sweet spot.

Rears can't go in back wall (too close to couch) and one side wall is an outer wall, don't want to mess with that. Actually I think the best option and best sound will be in the 45-degree portion of the ceiling above the couch. Spread out not too far, maybe halfway between the center seat and the outside L/R walls, also using aimable mid/tweet components so can aim mostly toward the couch. They will end up about 5 ft above my ears and 5 ft to either side.

I have an older 6" Infinity powered sub in the room already; this will complement the new speakers and we'll see how it does. Can always upgrade (or add another!) later.

Now for HARDWARE SELECTION....
Bought the TV earlier this year, its a 55 inch Sony Bravia KDL-55700B smart tv. Best Buy for $800, and I gotta say it looks terrific, haven't even calibrated it yet. It doesn't have the fastest processor, but plenty of options and is almost too large for the room. Wife would say definitely too large ;-)

As mentioned in OP, gotta go with in-wall speakers due to space constraints with the room being multi-purpose. The more I studied the options, the more I saw that there seem to be tons of open-backed in-walls, at reasonable prices. And there were some Enclosed in-walls, a few were VERY cheap and then the high-end enclosed. I ruled out the open-backed types once I studied the recommendations above and the sound science behind them.
I did not want bottom-market enclosed either... Luckily I have a friend who is a Monitor Audio dealer and can offer me some deals. I did like the SoundFrame series (SF1, SF2, SF3) which can be configured to mount IN or ON wall (this appealed to me since mounting ON wall is easier, and more reversible down the road). But even at dealer cost these were more than double my original budget.
So I looked at the CP Controlled Performance line. After tons of research and talking with my dealer buddy and the Monitor Audio rep, I took their advice and decided on the CP lineup. I finally placed an order for
Left/Right: CP-WT380IDC x 2
Center Chan: CP-WT240LCR x 1
Going rectangle for the rears!: CP-WT260 65w x 2
And the L & R are backordered!!! Grrrrr...... Having them wait on the entire order until they can get it all at once, don't want to risk getting half the set and not the L & R for some reason.
Even these speakers at dealer cost are a few hundred over my original budget. But I am ok with upselling myself, after not finding anything that I was happy with for less.

I also picked up a 100w Denon AVR-X1000 5.1 factory refurbished from Accessories4Less. Price was good, but first unit had a bad HDMI card and would not send any output to the TV. Sent it back and got another with little drama, so only temporarily annoying.
The LRCs above are rated for 100 watts and the rears for 65w, so this should be about perfect for 5.1, and more than enough for my smallish room.

EXECUTION
MA estimates mid- late-August for my backorder. I can live with that as long as they don't keep pushing that out. Meanwhile I think I am safe to hang the TV on the wall, I don't think there is anything that could change my mind about where that will go.
I can cut the holes for the wiring and mount those faceplates to make it look nice. One behind TV and the other behind computer desk, wires running in attic unseen.

Then when the speakers get here, I will be able to get the center channel in first, centered with the TV. Then the L & R based off those. Will be a snap to run the wiring from the computer/AVR desk to those via the new wiring channels.
Running wiring to the rears will be a bit more complicated... They will also enter the attic space behind the front wall, and hopefully up and over the vaulted ceiling, to the rear attic space. Then just up into the ceiling/roof space to the rears in the 45-deg ceiling.

I enjoy designing and executing this kind of stuff myself, but do rely on others with more experience for guidance. I find myself pushing my comfort zone limits a bit, and so thank you for participating in this forum and sharing your knowledge!

TV hung. Paper templates everywhere while I fine tune the layout.

Monitor Audio speaker pics...
This is a Left/Right: CP-WT380IDC. Note the entire midrange / tweeter assembly can be pivoted. That's a sticker on the center covering the tweeter :

Made one mistake installing the L & R. I didn't leave enough space on one side for the mounting tabs to swing out. Had to spend an hour carving out notches in the wall stud to accommodate them. Otherwise came out fine.

I pivoted the L/R mid/tweets inward and slightly down, right where my face will be when sitting. Great feature as you can't toe in a in-wall mounted speaker.
Same with Center channel, just aimed it up a few degrees and it lines up with same target seat.

I have since routed the TV power cord through the wall same as the input cables. All TV and speaker wiring runs through attic to access port under the computer desk.

With grilles:

Rears went in 45-deg ceiling above the couch.
I ran the wires from behind computer desk, tucked under the molding at the floor, under the attic door, up a stud in the attic to the speakers. No Problemo.

Also decided to turn them upside down, to keep the tweeters closer/more in line with the listening seats. As they are aimable as well, they do point nearly at the couch.

They work really well in this location. Key scene in The Matrix when I tested out the new system: after they removed the "bug" from his navel in the car, the outside thunderstorm rain and thunder effects.... awesome!

Post-op:
Well I am glad I researched and took plenty of time to put this together. I am able to run the TV as a second monitor, play a dvd / movie on my pc, or just send the DVR cable box signal to it. Or a combination if kids are watching tv while I "compute". The DVR cable box passes through when the AV receiver is off.

My old 6" Infinity sub is located under the computer desk near the corner of the room; it's actually doing just fine for now. No it can't shake the whole house, but I only need it about 50% power to compliment the system for this smaller room. For music its balanced well at least for level; def not the punchiest / tightest low end. Movie soundtracks seem to carry much more bass and I actually had to turn the sub level down a fair amount for Mad Max.

Running everything off my pc, then to the AVR, adds another layer of complexity with all the Windows settings. I do have to constantly switch my 'playback device' in Windows between my headphones (plugged into pc) and the receiver.

I would say the most confusing thing was figuring out my Denon AV receiver, in conjunction with the pc settings. I played with separating the video (hdmi) and audio (spdif) from the pc, but this gets really confusing and not all sources allow it, so you have to keep messing with it. In the end I just set it to send all via hdmi.

Only thing I have left is to try the Audyssey setup procedure. For now I input all the distances and levels of each speaker manually into the AVR.