No dropouts since setting up the USB / Ethernet  rig. I'm giving it a couple more days to make sure the problem's solved before changing anything else.
 
Still getting DPC latency spikes, but no audible artifacts, just as before.
 

Well that sounds like good news.

Just a follow up in case anyone is interested.
 
Using this setup:
 
1. short USB cable to USB/Ethernet adapter
2. short Ethernet cable from adapter to female-female Ethernet wall plate
3. 50' new Cat6a cable to Ethernet/USB adapter, which plugs into computer
 
I was getting no dropouts at all, but a crackling noise appeared after playing back any file for more than a few minutes. This happened with any audio software, not just SONAR.
 
I substituted a new high quality 12" Cat6 for the short Cat5 cable I had been using. No difference.
 
I substituted a 25' Cat6 (not Cat6a) cable for the 50' Cat6a cable. No more crackles.
 
So for anyone with a USB cabling issue, I would recommend the USB/Ethernet adapter solution. The one I got was made by IOGear and cost about $60 at Newegg.
 
I haven't done any more testing, so I don't know exactly why replacing the long cable solved the problem. I see three possibilities:
 
1. The IOGear specifies Cat6. Could Cat6a be incompatible? (Probably showing my ignorance here.)
2. 50' was too long and 25' is OK. I doubt this, because you are supposed to be able to get USB connections over 160' with the USB/Ethernet adapter.
3. I had the excess length of the 50' Cat6a cable wrapped into a fairly tight coil, about 8" in diameter. Since there was about 35' of excess, this was a lot of cable in a pretty tight coil. Inductance? Who knows.
 
Since my problem is fixed, I probably won't do any more to find out precisely what happened.
 
Oh, yes, the latency is still with me, but no audible problems. And I still hear that noise from time to time, but it doesn't affect my audio at all. Zero dropouts now.
 
I hope this is helpful to someone.

Could be inductance if the cable's coiled I suppose, though I remember having problems with a short Cat5 cable a few years ago. Only three metres or so used to connect a PC to the router so the router could be administered by ethernet rather than wi-fi. Kept getting disconnections between the router and that PC, and changing the cable for another one that was the same make, same length, same everything solved the problem.

I guess not all ethernet cables are made equal and every so often one isn't up to spec.