I think the main fix came from the driver in the Zuma' network adapter (I thought this was current but some digging revealed a later version from Intel).
Bingo.
Had I seen this thread earlier I would have pointed immediately to the Zuma's network adapter driver. I have seen a lot of network problems with the sort of symptoms you described. Getting the latest Intel driver has almost invariably fixed the problem, especially when a third party (motherboard or network card manufacturer) driver was in use initially.
It is also common in my experience to require several reboots of a PC before a new network driver settles down and works reliably. Well, at least two. Usually this is because files were in use requiring a reboot, and then after the first reboot the driver wants to make more changes, and requires another reboot. Behaviour depends on whether you download and install a complete (large) driver package, or a driver installer package that then wants to download and install more components. Sometimes it needs a reboot to complete negotiations with other network components for speed of connection.
Now that you have the Zuma adapter set to negotiate LAN connections automatically, you may wish to check what speed your Zuma is connecting to the router. You can check that using the router interface, or as I prefer go to the "Control Panel \ Network and Internet \ Network and Sharing Center \ Connections" and click the "Local Area Connection", and probably in the Intel driver dialogues. The real speed of the connection will be listed. It should be 1Gbps on any modern network adapter.
However, as you have an Acoustic Revive LAN filter on that cable, I suspect you will find that it running at 100Mbps. I tried to find specifications for the Acoustic Revive LAN filter but could not find any. What I did find was one person who said that it only supported 100Mbps, plus someone who reported it only had 5 of the 8 wires connected in the Ethernet RJ45 connector. Check yours. If it only has five pins connected to wires in the connector, then it only supports 100Mbps. Based on he fact that the Acoustic Revive LAN filter couldn't be seen when you set the Zuma Ethernet adapter to 1Gbps, I am going to assume that it does only support 100Mbps. (It wasn't the full duplex that stopped this connection, it was the speed the filter supports.)
What I suspect is that you have had network issues for some time, resulting in lots of dropped Ethernet packets and hence, resending of packets and a subsequent reduction in the real speed of your Ethernet connection. As the cable is connecting your Synology NAS to your Zuma, via the router I assume, I guess it could effect JRiver playback of the files by causing JRiver to wait for data, though this shouldn't happen because the data would be buffered at the Ethernet receiving end. If the network connection was bad enough though, the buffer could empty, and that would definitely cause audible problems with playback. The fact that JRiver was unable to find files on your NAS while running automatic import would suggest that you had very significant network problems.
If you had the network configured to "Auto" speed connection it would have been running at 1Gbps by default. When you installed the Acoustic Revive LAN filter into this data path, the network speed was renegotiated down to 100Mbps and the reliability of the network connection would have improved dramatically. That may have been why the Acoustic Revive LAN filter had such a positive effect on the listening experience. (Yes, I am a Acoustic Revive LAN filter sceptic. Digital data is digital data. Noise on an Ethernet cable doesn't effect audio playback. However, a poorly performing network certainly does.)
Now that you have fixed, or improved, the network reliability at the Zuma end you may wish to check that you are using good quality CAT6 or CAT7 Ethernet cables, and double check that the Synology NAS has no outstanding Ethernet adapter issues, and make sure that new router has the latest firmware with no issues. You may even wish to find a utility to check if you are getting dropped packets at the Zuma end of the connection. Or you could just run the "Ping" command in a Windows CMD window from the Zuma to the NAS.
If all is good you may wish to try removing the Acoustic Revive LAN filter and test the listening experience with the network running at 1Gbps, then separately at 100Mbps. Your router, set to auto speed configuration, will negotiate the connection speed to whatever the Zuma network adapter is set to, when the Acoustic Revive LAN filter isn't inline.
While even a 24 bit 192kHz stereo LPCM sound track isn't going to saturate a 100Mbps Ethernet connection, other activities such as copying files to and from the NAS, importing new music, audio analysis and any other library maintenance functions would benefit from a 1Gbps connection, so it might be worth trying to live without the Acoustic Revive LAN filter. Unless, of course, the Acoustic Revive LAN filter does work at 1Gbps, and it makes the music sound better for you.
I would also convert back to using UNC paths. Much more reliable and easier to maintain. Once the network is working properly.