Well, the OP pretty well sums it up.
Basically, there were four types of failure noted.
High res playback failed after a variable period of time when playing from the Linux RoonServer to the Squeeze endpoint. This is the central issue, as far as I’m concerned. It almost felt like it could be related to something that gets cleaned up by the overnight cron jobs. The “first” session of a given day would play longer before failing than subsequent attempts. After one failure to noise, an attempt to play a track would only last a minute or so. As far as I can tell, there were no events associated with when the stream failed.
The “limit sample rate” dialog in Roon’s settings for the Squeezebox only offered options up to 48K, not 96K, as it should have. I discovered this while looking for a temporary workaround for the underlying issue. No idea if it might indeed be a symptom of the main issue. But it certainly feels like it could be. In any case, this isn’t the swamp we set out to drain.*
The queue view (as toggled by the little icon in the scrubber bar) fails to show any tracks in the queue if an album is playing from the Linux server. From the Mac version of the server this feature behaves as expected. I didn’t put any real effort into trying to isolate this. I just noticed it. Related or a separate issue? I have no idea.
No audio when playing to a Squeezebox 3 endpoint. I never checked to see if the Mac version of the server behaved any differently. It’s the least of my problems, frankly, but who knows, it might be related.
When I played the Linux server to a Roon endpoint instead of the Squeezebox Touch, high res files played correctly.
The “limit sample rate” dialog appeared correct, but since it reflected a different end point, I doubt that means anything useful.
When the Mac server is in use, all the features - high res files, the sample rate dialog and the queue list appear to function as expected. I didn’t check the function of the SB3 with the Mac server in play.
Squeezebox has a function to check network throughput to the endpoint. (LMS-to-SB) I used that and there was enough bandwidth available for a 24/192 FLAC stream, but not a lot of headroom. Given that 24/192 works Mac server-to-Squeezebox (barely), Linux server - to - Roon endpoint, and LMS-to-Squeezebox, I’m disinclined to think that we’re seeing networking issues. (The Mac server is running on a laptop, so there are more WiFi hops when it’s in use. I would expect less throughput in that case. But I can probably lash up a wired connection to the same gigabit switch the Linux server and the data store are on, if need be for testing.)
*I’m from Florida and we now know that draining swamps is not usually a good idea. But I like that expression anyway.