One option is to put your Play:1’s on stands for the rear surrounds, as shown in this Dolby Atmos setup image below? Maybe have their power cabling under the floor, or under nearby seating (perhaps?) or just have them quickly and easily moveable.

Otherwise it’s the Sonos Amp and two rear ceiling surrounds, I guess, which will be okay for HT rear surround-audio anyway, if you prefer that option, but ensure your Amp is located nearby and preferably in the same room as your Sonos Arc, so they can connect wirelessly over a 5Ghz Ad-hoc connection.
 

Thanks, Ken. I absolutely would do that. My problem, such as it is, is that I have a truly understanding wife who supports my home theatre needs, but three boys who are 5, 4 and 9 months, who think it would be great to use a stand as a sword or support for learning to walk. I will probably be there in a few years, but for the next few, that’s unfortunately a non-go. 
 

I was thinking ceiling would be better than L/R rears for this, which is another option.  Sort of ironically, even if I went full 5.2.2 (or 5.2.4) with built-ins, I’m not sure I would do much better for a soundstage, given the open back. 

In that case, I think the ceiling speakers are the way to go here - they should be okay for rear surround audio, iMHO.

It’s when folk try to use ceiling speakers for the front left/right HT channels that I’m personally not a fan of that idea, but rear channels should be fine.

Also, here are a few pics of the room itself (ignore the fireplace, that’s a wholly different A/V fight that I’m breaking about even with the better half on - essentially we are building a wrap around mantle and I can have a spot for the ARC on it if I need it; though Tv will still need to be angled down and above the fireplace, but at least insulated from the chimney heat). I’m just trying to decide if I should stick with the ARC here with ceiling rears or just abandon and go to a traditional setup and conquer the question of where to place a center channel. 
 

Note, these are staged photos from before our purchase, so disregard furniture, etc  

So it’s rather a high ceiling and angled too, if you can’t go with speaker stands, it’s your only option to use the ceiling for the Sonos L/R rears. However…

I’m not sure the Sonos Arc will do its job here either with such a high and angled ceiling and where you intend to locate the TV.

Yeah, nothing about it is great for sound (though the space itself is wonderful!). What troubles me most is I’m not sure it is great for a traditional system either. Would you think I would do better with a traditional setup, still using ceiling wounded rears?

sorry, obviously this isn’t anyone’s problem but mine, just appreciate anyone’s thoughts that isn’t trying to sell me something. 
 

thanks again!

Whilst I’m also not keen on TV’s set above fireplaces, I think if that’s where I was going to put a TV anyway, I would maybe go with two Sonos Amps and perhaps two Subs instead.

One Sonos Amp to drive the rear ceiling speakers (as mentioned) and another Amp to drive two floor-standing front speakers and have a pseudo (phantom) center channel. I would also use two Subs in an open room that size, placed on either side of the room.

There maybe better options perhaps with other (wired) HT audio equipment, but I’m looking at this from a Sonos perspective only and it mostly being a wireless setup in the room.

I’m not keen on ARC in this installation. I know that there will be a push for TV over the mantle. This is not a great idea because it will become uncomfortable to view after a while. There are motorized mounts that can drop the TV below the mantle, but it would not be a great idea to drop the TV in front of a roaring fire.

Can you mount surround speakers low in the ceiling even with or behind the viewer? The bookcase would be an obvious location, but I’m not certain what the other side would be.

Even if you arrange childproof speaker stands for the surrounds, there will be wires to run.

If that is a real brick fireplace, running wires to a TV will be a challenge.

Without any padding (carpet, drapes, etc.) this room will be an echo chamber and you’ll probably be watching TV elsewhere.

If you can find someone familiar with Amina speakers, speak with them. I know that there are a bunch of cheap, low class similar looking products, but Amina is different. These speakers are first class and can handle very difficult situations such as this. At first blush they seem to break every familiar speaker rule, but there are some not so familiar rules. Hopefully you can find a demo.

@Ron Panasonic,
Just to also add, I agree it’s a real lovely (wonderful) room, but as you mention it’s quite a difficult one for a traditional Sonos HT setup.

The two Sonos Amps HT setup would be my choice, with either one, or preferably two, Subs, but maybe others here in the community will throw in their suggestions and give you some other options to consider. 

Edit: I see @buzz has now posted as I was typing this post, so there are other options to now think about - thanks Buzz.

Thank you both very much! Buzz, with the respect to the Aminas, I know Sonance has a somewhat similar product, but it seems to me you may have some experience with folks at Amina specifically?  I know the wife would absolutely love the architectural aspect of fully disguised/hidden speakers here.  Are the animas directional? I will try to connect with someone there. 
 

Frankly speaking, the wiring issues you both mention is what made me think that an Arc and amp may be the best way to go (perhaps trying to get one of Samsung’s singular wiring solutions - ie, HDMI (eARC) and power through the low profile wiring may be a way to handle). I have a bit of flexibility with the new mantle we will build and can wire through that - it is a real brick fireplace, but we can insulate the “mantle”. 
 

ken, I like your suggestions, I just struggle with a phantom center - that system would sound great for everything but TV/movies, where I worry about muddled dialogue. 
 

thanks again for responding to a noobie on Friday, it’s good to talk through with folks who are of similar mind, but without agenda. 

Sonance is not in the trash group that I mentioned, but in my opinion the Amina’s are in a different class. I don’t know how one would find a demonstration where it would be possible to compare both.

The Amina’s are not directional in the traditional sense. You are very aware of the musical elements, but not very aware of the exact speaker location.

In my opinion, if you are using traditional L/R speakers that image well and are close to the TV, center channel is mostly redundant, however, some A/V kit might not handle this as gracefully as we’d like. It is important that the L-C-R speakers have the same timber. If the timber is different, an actor’s voice will change while walking across the stage. In your fireplace mounting scheme, I think that the L/R speakers will be too close to the TV, relative to your viewing distance. You’ll have OK sound, but not much of a ‘grand’ surround experience.

Those marketing photos distort the room dimensions for me, but I would go large for the TV. Make a statement.

Buzz, thank you for the thoughts. And thank you, also, Ken for yours. 
 

buzz, to make sure I understand your thought right, are you saying that regardless of whether one would use the LCR front speaker set up, it would inevitably get muddled with the work-around for the fireplace mounting (ie, sound muddled regardless if you didn’t use a dedicated center channel)?

im obviously imagining something here with no actual regard to budget (and I can’t blow, like, $20,000 on sound in this room), would you think you could make it work with say animas in the traditional 5.1 setup (or maybe even 4.1) - and I like Ken’s idea of going for a 5.2 or 4.2 set up with subs - with in-wall fronts and ceiling mounted rears (and I guess open up a 5.2.2/5.2.4 or 4.2.4) system if I win the lottery)? 
 

I guess that feels like I am forcing a square peg into a round hole and would be better, which I think you said originally, picking a different room for the non-casual viewing. 
 

thanks again, guys, really do appreciate the thoughts.