Will new Sonos ARC provide full featured Dolby Atmos quality sound when connected to: 1) ARC-enabled Samsung TV (Q9; late 2018 edition); and 2) latest Atmos-enabled 4K Apple TV?

Yes the lack of robust interoperability with the ATV might be a deal breaker for me too. Not sure the ROI will be there unless something changes. Of course a new ATV is rumored to be released in 2020 so who knows what that will offer. 
 

finally anyone know anything about a rumored Samsung firmware update enabling eARC on recent ARC TVs? Probably asking too much but I’ve got a lot invested in this fairly new tv model.

I’ve not seen any rumors. And since it would require new electronics, it’s hard to believe it could be a software update. 

I have a 2018 Samsung Q9F, an Apple TV 4K, and an Arc. I get same issues as noted above, that the Samsung will output only PCM to the Arc and the option to select Dolby 5.1 or Plus is greyed out. Changing the audio output settings on the Apple TV 4K seem to have no effect on this. I did read  that Samsung seems to be updating the software on some older TVs so that the HDMI ARC port works like eARC where the underlying hardware is capable of supporting that. I don’t know whether this will happen for the 2018 models or not. In the meantime, I’m considering trying something like this (https://www.amazon.com/HDMI-Splitter-1x2-4K-60Hz/dp/B07VP37KMB/ref=pd_sbs_23_5/130-9843274-2515207) to split the signal coming out of the Apple TV 4K so that the Arc gets ATMOS and the TV gets Dolby Vision. I should add that I haven’t tried the optical to HMDI out from the Samsung TV to the Arc.

The AppleTV uses LPMC to pass high definition audio. At a guess Your TV would probably down mix it to PCM Stereo worst case. There are similar reported issues on other TVs. LG have released new firmware that enables LPCM pass through on the C9 via eARC. Once the Sonos Arc gets updated for LPMC in the future than at least on those TVs the problem will resolved  

Potentially your TV might be able to pass DD + from the internal Apps. You would probably be best to contact Samsung directly to understand the pass through capability of your TV. 
 

Splitting the hdmi audio and video and trying to feed the audio into the Arc will not work. The Arc is expecting an ARC/eARC input and this uses different audio pins compared to the standard HDMI audio. 

Hopefully Sonos will prioritize the LPCM update.

I have the same setup as you.  Sadly, I’m not confident that we will ever be able to have ATMOS, or even DD+, from external devices (such as the ATV 4K) as it seems passthrough will not work.  I’ve seen some reports that Netflix on the internal app may pass ATMOS.  But my family is very comfortable with the ATV interface and that is what we primarily use for content, so don’t think it’s worth it for me to switch to another streaming solution just to get ATMOS on one service.

I’ve held off on pulling the trigger for the Arc until I saw some test cases with my setup, and looks like I made the right call for now.  I’m sure it’s a solid step up from my current Playbase/Sub combo, but for now think I will stay put.  But fingers crossed for a software update that will magically enable passthrough!

And kicking myself for not getting the LG as I was on the fence...

I installed my new Arc, sub and a pair of one's yesterday. Connected the Arc to my 2019 Samsung Q90r TV with the provided HDMI cable. Have an Apple TV 4k connected to the TV as well with this HDMI cable. https://www.amazon.com/gp/product/B07NXPS94Q/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I can confirm that I'm not getting any Arc content from Netflix or iTunes Movies via the Apple TV 4k. On the embedded apps on the TV, I am getting Atmos from Netflix. I'm not able to get it from the iTunes Movie app on the TV I think because Atmos isn't supported in the app. Haven't tried Amazon Prime yet.

I've seen the talk of a firmware update potentially bringing eArc to this TV - definitely hoping that occurs. Also talk of a new Apple TV coming soon - perhaps it will handle Atmos differently and that will fix the problem. 

I tried the built-in Netflix app this evening and was able to get DD+ on the Arc. Whether the sound options are greyed out in the TV sound setup or not does seem to depend on what the current stream is presenting. Since my Q9F supports ARC but not eARC, I don’t think I’m getting Atmos even via the TV’s built-in app, just DD or DD+. Once the ARC supports LPCM, I think using a splitter from the Apple TV 4K to the Arc and TV ought to do the trick.

@jam9 I have a similar setup to you and been struggling to get anything other than 5.1. Can I ask what film/program you used on Netflix to get DD+?

Many thanks 

I hope you’re right, but I believe that a splitter will not work because the Arc needs to receive an HDMI ARC input.  I’m not familiar enough with LPCM, would that change this requirement?  I’m not even sure that the Q9F would pass LPCM, at least from extrernal sources.  But, would love for someone to test this once this capability is enabled!