At present, the only Atmos-capable rears are the Era 300’s as no others have upward-firing speakers. 
 

That said, I’m not sure how you could get Atmos effects from in-ceiling speakers. 

The best location for Amp based passive surrounds are in-walls, at ear level, behind the main viewing area.  With an Arc, this will give you Atmos effects through the Arc only, but the surrounds would not be clashing with the up-firing Atmos speakers in the Arc, as they would if they were in-ceiling surrounds

Quick and rough sketch of the room.  So from your comments, looks like i have very limited options. Am i understanding correct then if i place speakers in ceiling at  A B they will just act as left right rear speakers, i could then leave the arc sound bar to do limited version of virtual  Atmos ? 

That is correct.

I’d seriously look at a pair of 300s behind the couch. If you feel the stands are not dog-proof enough it is easy to add more stable bases, even ones that extend under and anchor to the couch.

Hi

You aren’t as limited as you may think. The Arc will produce Dolby Atmos effects as a standalone speaker as it can simulate rears. However, in you case adding the sub will also improve the overall effect. I recommend the full-size Sub Gen 3.

The in-ceiling speakers may work to your advantage. When playing music they can be set to full range (stereo) for better enjoyment when in the kitchen area. After music is done they will auto switch to surround mode. However, as has been said they will not produce Dolby Atmos effects.

For in-ceiling speakers I recommend the package in the link that will integrate quite well with the Arc. You also have a choice of 6 or 8 inch speakers to suit your budget or just bigger sound. They can also be tuned along with the Arc using Trueplay (requires an iOS device).

https://www.sonos.com/en-us/shop/amp-and-ceiling-set

we have a great dane so ultra dog proof required one slight bump or excited tail wag ….

I would say your only choice is an Amp w/ ceiling speakers.  You will still get Atmos, and the Arc is pretty good if you have a flat ceiling between 8-10 feet and sit in the sweet spot.  I’ve only got Ones for rears on my Arc, sit 11-13 feet away, with a flat 9 foot ceiling.  Was watching 3 Body Problem  on Netflix last night and the Atmos was outrageous (Episode 3, anytime the Civilization was terminated in the “game”).

Thank you, I would like the 300 but practical reasons i dont think i will go there. So two ceiling rear and the arc it will be then. Thanks for all responses.