Thanks to all who responded.
soix: We have a (Google) mesh network in the house but most places I've read say Ethernet is preferable.
antigrunge2: I've ordered one of those cables for the short run between the EtherRegen and ROCK server. Certainly worth a try!
gdnrbob Excellent idea; I'll check out HiFiShark. Didn't know about that site.
jerrybj: Running fiber to the streamer presents the same logistics problems as the long ethernet run, which I can handle, but would also necessitate an FMC near the streamer, with either a wall wart power supply or another linear power supply.
audphile1: I'll check out Network Acoustics Eno; that's a new one on me.
If anyone's interested, here's the setup in the hallway outside my listening room.