Surround Sound in room with 12' Ceilings and limited space behind couch

I'd try just setting 300s on that short wall as a first option.

They do make long-drop ceiling mounts, some pics in other roo  setup topics if you go look.

You can adjust the height channels to offset (some/mostly) the high ceiling issues.

To start with, you could do what Stanly_4 suggests. I would consider adding a shelf between the wall and the sofa; this would also hide the cables and allow you to add a charger for a phone or iPad, or perhaps place a small lamp. You can position the shelf however you like and in whatever width suits your space, the image is just an idea.

I wouldn't worry too much about losing the overhead sound effects, as the speakers are positioned quite close to you—you’ll hear the sound before it dissipates toward the ceiling. Furthermore, the Arc Ultra also projects overhead sound, which you will also be able to hear.

Hiding the power cables can be done (fairly well) with a cable-race put on under the under-shelf trim. 

IMO setting the speakers on a faux shelf places them too low. The wall behind also allows them to be placed further outside the seating area. 

Hiding Cables aside I wonder if there are receptacles behind the sofa? That may change recommendations. Maybe the OP can clarify.

If there is power behind the sofa some through-wall power/cable pass through plates would be a good option. Usually just cut the mounting hole and run wires inside the wall. Usually a handy-homeowner skill level job.

Many options

https://www.amazon.com/Management-Resistant-Receptacle-Relocation-Nightstand/dp/B0HKFRZXC4/ref=sr_1_5_sspa

https://www.amazon.com/VCE-Single-Voltage-Mounting-Bracket/dp/B07QJXRRFM/ref=sr_1_26

A lot of speculation about power being behind the sofa given that the other side is a stairway. However, given that the OP is considering surrounds the possibility of power being behind the sofa is 50/50. However as I said it would be nice if the OP would respond and clarify.

A follow up question, I am having some low voltage work done to run ethernet to several more places for outdoor cameras / APs. Is there any value in getting speaker cable run to the ceiling above the sofa or elsewhere in the living room, or would it not be doing much on top of the soundbar + sub + era 300s?

Overhead speakers are a nice option. Using Sonos Amp or Amps can integrate them into your Sonos system. However, I’d only recommend them for music.

Some use them as surrounds in a Sonos Home Theater setup however, the positioning has to be spot on. Also, re-arranging seating can be difficult to get the same surround effect as with the initial setup. You should install wall mounted volume controls for audio versus using your phone or tablet for better convenience. 

Sonos Amps using Sonos In-Ceiling Architectural Speakers by Sonance can drive up to six speakers.  If you want in-ceiling speakers in different rooms to play different music (for example three in room A and three  in room B) requires multiple Sonos amps.

In-ceiling speakers can be grouped with a Sonos Home theater or other Sonos speakers. When grouping with a Sonos home theater it’s not recommended for TV Audio as an echo effect will be caused by a 75ms delay. Grouping in-ceiling speakers with a Sonos Home theater for music is OK; as everything plays in sync.

Learn more about Sonos In-Ceiling Architectural Speakers by Sonance and  Amps here. *

 * There are also Sonos In-wall Architectural Speakers by Sonance

EDIT:

Using In-ceiling speakers as a supplement do nothing for a Sonos home theater setup. The max number of components in a Sonos Home Theater is 5 :

• Soundbar
• Surrounds x 2
• Subs x 2 (Sub x 1 may be adequate in some environments)

I have a similar room with high ceilings, and I went with the Arc Ultra and Era 300s on the short wall behind the couch, which worked well for me. I used the height channel adjustment after running Trueplay, and the overhead sound came through much better than I expected. I kept the in-ceiling speakers for music in other rooms instead, as they did nothing extra for the home theater setup.