best speaker layout

That is a difficult space, the Arc Ultra and Sub 4 will do fairly well there, the ceiling may make the height channel a bit weak but still very listenable. Mine is about as bad and I'm happy.

The rear though, ouch, maybe ceiling mounted 300s? 

How much space is on the far end between the wall and the couch? If the same distance as on the near end you may be able to use Sonos speaker stands. 

My Era 300’’s are about 6 - 8 inches from either side of my seating.

To help make a determination on the stands for either the Era 300 or 100 here are the dimensions taken from Crutchfield a respected authorized dealer in the US. Sonos is not as detailed on their official website which IMO they should be.

The most critical measurement is the Base. The speakers are not as wide. Although Sonos does provide speaker dimensions on their website in the online manual under FAQ’s for each.

Looking at your room, I’d go with Era 300s on stands rather than wall or ceiling mounts. With the couch against the back wall, placing them slightly wider than the seating position will give you the best surround effect and make Atmos content much more immersive.

Personally, I'd start with stands. In rooms like this, a few inches of movement can make a surprisingly big difference, so it's nice to have the flexibility to experiment a bit. I wouldn’t overthink a 7-channel layout in that space. Arc Ultra + Sub + Era 300s is about as good as Sonos gets for a room that size. If you want to play around with positioning beforehand, something like this can help visualise speaker coverage, but I think you'll get great results with a fairly simple setup here. The room actualy looks pretty well suited to that setup.