The Arc should be doing Atmos with no problem, not sure why you aren’t getting that. More details of the problem would help.

The Amp can bond with the Arc for the surround channels.

https://support.sonos.com/s/article/4717?language=en_US

https://support.sonos.com/s/article/2927?language=en_US

https://support.sonos.com/s/article/4807?language=en_US

I’d suggest ear level in-wall for the surrounds, having them too high does not help the surround experience.

Not sure this would work well. You’d use the Arc for front and surround would come from  the Amp, the same sound (meant for only two rear speakers) from both ceiling and walls? Would that help? Would it no emphasise  the rears too much?

I would certainly see no harm in setting the ‘bonded’ Amp to output rear-channel TV audio to both rear and ‘above-head’ Sonance speakers. It ‘might’ be overkill for the rear-channel TV audio and maybe a little strange having it coming from both behind and above, but in the Sonos App there is a ‘TV level’ slider bar in the ‘Surround Audio’ settings for a Sonos HT setup, that can be adjusted to shift the volume ‘balance’ between front and rear-channel audio and I ‘think’ sliding that towards the front chancels might assist after doing the setup. It maybe the case that Trueplay will help anyway in this regard too.

It would certainly be ‘room filling’ for music audio.. but I’ve never tried this kind of setup, personally speaking to see how it actually may sound - I’m not a huge fan of ceiling speakers for general music playback, although they’re okay for background audio, but nothing ventured etc. 

Also if it does not pan out to your own liking, you could always go onto add a second Amp perhaps and create a second ‘Sonos Room’ for the ceiling speakers and then use the two rooms grouped/ungrouped for different types of music playback instead.

Ya my thinking for using them as rear surrounds on Movie, Show, Gaming content would be to see if TruePlay would work well enough to make it, so the rear sound is blended between the 2 levels (in-wall and in-ceiling). So instead of having the sound come from 2 exact points, they would instead create a more of a rear surround blanket.

For Music I do see the appeal since it would just be more room filling, but music playback won't be primary use for the setup.

I think no matter what I will do in-wall to be closer to ear level for surrounds since its just standard 5 channel, but I am leaning to trying out adding the ceiling ones to see what it can do :)

TruePlay would only work, I believe, if the speakers were the Sonance Architectural speakers. As far as I’m aware, they’re still the only ‘in-x ‘speakers that can be TruePlayed. Other Sonance speakers, and other company’s speaker do not yet have that ability. 

Speaking personally, I don’t believe I would appreciate having rear surrounds coming from four speakers, rather than two as designed by the folks doing the audio design for content. Too much muddying of the directionality. For music, it would likely be much better.