I’m not overjoyed to answer this, it is a challenge Sonos is not designed for, and I am not sure my words will help, but here it is. 
 

1. Possibly. ARC is fairly low amperage, if you go with such a device, I’d certainly want it powered, to ensue both devices get a ‘full’ power signal. I think you’d want to leave both ‘rooms’ in the Sonos software to recognize incoming input from the HDMI ARC signal, so there wouldn’t normally be an issue
    
2. Not sure what benefit a Port would add to this, unless the in ceiling speakers have an amplifier that needs to be fed. And any Sonos device, including the Port, would be subject to the delay when grouped with the soundbar. The Sonos Port can’t drive surround speakers, but it really is unclear if you want these in ceiling speakers to be surround, or carry the ‘front’ signal.
    
3. I’d be tempted, before going to the lengths of installing the ceiling speakers, to get a small Sonos device, like the Sonos Roam, and test that in your kitchen. By definition, it will be set up as a different ‘room’, but could play the TV’s channel, with the delay, if you group them. 
    

Not sure I answered all your questions. But hopefully this gives you something to think about, and expands your knowledge about what Sonos can, and can’t, do. 

​@jaspass 

If I got it right, you have set up two „rooms“ and like to group or ungroup them from time to time.

For streaming sources that’s no problem but for tv audio source you get that delay. 
I recommend using the „tv dialogue synchronization“ option within Arc Ultras settings. For me level 2 or 3 helps to get audio of both rooms in sync without noticeable delay. 
Be aware that this option causes a minimal delay between video and audio from tv source, but imho that really isn’t remarkable. 
 

You can control both Sonos rooms from Arc Ultras supported voice assistants. If Arc Ultras microphones are too far away from dining room, you could use a cheap Amazon Echo Dot over there. 

Thanks, yeah I get the feeling this is not a straight forward thing to answer, but you never know how creativity can solve for this is my hope.

Thinking about the first option discussed, I am realizing an HDMI/ARC output from a tv cannot really be split as it is meant to be point to point.  However I am wondering (just thinking out loud) as it is easy to split a digital optical output, then use the Sonos optical adapter for input into the Arc Ultra, and the other as input into the Sonos Amp.  Maybe that would work?  Anyone heard of this solution being successful?

​@jaspass 

I don’t think splitting the tv out signal is a good plan. You would need a long optical cable connection and maybe a signal amplifier. And I doubt the result will be synced audio without any issues. 
Why don’t just try the easy way I mentioned above. I do it that way with my tv setup and a single Era300 in the dining room. 

Also, optical is limited to 5.1, and doesn’t have the bandwidth available to carry an Atmos signal. 

​@Airgetlam 

Thanks for the feedback!  Sorry when I replied I hadn’t seen your message yet.  So you are saying the TV Dialogue Sync option gets the audio better in sync with other rooms, and the sync to video (lips moving) is hard to detect?  That is good feedback.  I can adjust that setting now without the other speakers just to see how setting 2 and 3 look/sound.

​@jaspass 

It was me… but no problem at all.

Give it a try and I‘m pretty sure it also will work for you. 

You’ll need a PORT and an amplifier from your closet or an AMP to drive your speakers. Either will incur the same latency. You can tinker with the impact of the latency (somewhat), at the risk of damaging the voice sync by adjusting TV Dialog Sync. Maybe this will be constructive, maybe not.

If the TV and the in ceilings are really that far apart. You might not notice the delay. I have an open plan kitchen. The One I have there is sometimes grouped with the Arc Ultra in the living room. The delay is hardly noticeable to me.

​@jaspass 

If you already have any other Sonos speaker besides Arc Ultra, you could try what I suggested before buying an Amp or something else.

​@Schlumpf 

Yes I was thinking about this as well.  Unfortunately I do not have another Sonos speaker as I am just getting into this system.  I pulled the trigger on the Arc Ultra first, and just figured I’d be adding the Sonos Architecture speakers to the room later, until I ran into the news about the latency which stopped me in my tracks.  Now I’m hoping to salvage the idea of keeping everything Sonos as that would be the best situation for the house given its simplicity and easy interfaces.

I do have neighbors that have Sonos, and maybe I could borrow one of their portable speakers to test this out as you outline.  At minimum without a speaker I can adjust the Dialogue sync today on my soundbar and see if that feels ok or noticeable.  However ideally I can get another Sonos speaker going to test before I pull the ~$1300 trigger on the new set of Sonos speakers and amp so I am confident in the purchase.

Lastly for context, the room is about 30’ long, which is ~27ms delay from source to back of the room (kitchen) where new speakers will be.

I would test whether you are sensitive to lip sync problems before trying the audio delay feature.  Download a copy of VLC and play a video.  In the settings menu for VLC, you can adjust the “Track Synchronization” in ms.  Set it to delay the audio by 75 ms and see if the lip sync is noticeable. 

Some people, like me, are very sensitive to lip sync errors, and can just about pick up the delay when Sonos is set at the default (~30 ms, which is supposed to be under the threshold where it is noticeable).  75 ms would (and does) drive me bonkers.  Others are not as sensitive, but try it out before you go spending any money on more Sonos.  

You can’t split an HDMI-ARC signal, you’re correct on that.  You CAN split HDMI signals though.  If your video source is a separate cable box/streaming box/etc, then you can split the audio signal coming off that with and HMDI splitter.  One end goes to your TV and Arc Ultra, another end goes to one of your old receivers that takes normal HDMI + passive speakers and maybe a 2nd TV (2nd TV may be required, not sure on that).  You could perhaps test this out to see if the audio is in sync enough to consider using ceiling speakers.  Both your Arc Ultra and 3rd party receiver will be playing audio as soon as possible, but they are not aware of each other and will not try to play in sync.

The issue with using HDMI splitter like this is that most splitters are setup to tell your source the lowest quality video source that the sinks (TVs) can handle.  So you don’t want to have a high quality TV connected to low quality TV (or maybe no TV) and end up with low quality image on your expensive TV.  There are some HDMI splitters that can downscale for the lower quality TV, but they are more expensive.

I do have a setup similar to this, but I am using multiple TVs of the same quality, each with thier own audio systems, in completely different spaces.  They are pretty much in sync, but hard to tell as they are not near each other.  I also have a setup where there is a 2nd TV (a monitor really) in the space space as my main TV that syncs with video great...but I don’t have audio system for the 2nd TV, so can’t confirm audio syncing.

Not sure if this makes sense, and I’m sort of skimming over details, but this is another option.

I wanted to drop a follow-up here to conclude a bit.  I decided to grab a Sonos ERA 100 to test before making the larger investment, so I went to Best Buy last night and purchased one.  Both the Arc Ultra and ERA 100 are connected via wireless wifi in my house, and were ~30ft apart from each other, and I tested the Samsung Frame TV as the source which is connected directly to the Arc via the HDMI/ARC port.  With no TV Dialogue Sync I could notice a slight delay between the speakers, it wasn’t horrible, but I don’t think I would want to have that all the time.  Ultimately a setting in the TV Dialog Sync at level 2 seemed to really smooth things out, and the video (to me) still appeared in sync with the audio.  In short, I fell much better about pulling the trigger on the Architecture speakers and amp.  Not only did I feel the room functioned as one single sound system sourced from the TV, but it really smoothed out the audio levels in the room allowing each speaker to function at a lower more even level.  It made walking around the room comfortable and consistent in sound volume.

Add to this I am very excited to see the new Alexa AI updates coming out as I believe it will make using this system even easier, and ideally will avoid having to use the Sonos app for most common functions (such as turning speakers on/off in areas, playing music and triggering songs/artists, etc...).

Thank you everyone for your feedback and thoughts, it was all very helpful!

​@jaspass 

Thanks for your feedback. I‘m happy to hear that you are satisfied with audio sync and also still with lipsync using the tv dialogue sync option. 
And of course Alexa KI will be a very interesting project, but I think Amazon will start this only for their own Echo devices. So I‘m not sure if, when and how they will support the KI feature for Sonos and other 3rd party Alexa devices. 
Just to control your system for the basic features imo standard Alexa or Sonos Voice Control also work very well. I won’t recommend starting any music besides radio stations (which almost use lower audio quality) via Alexa, because in that case the stream won’t play in best on Sonos available quality but in Alexa cloud mp3 quality.