After a brief moment of "Oh, Crap!" when both my WiFi and Ethernet stopped working this morning, it is now resolved.
WiFi mysteriously fixed itself after a third restart. Ethernet was resolved by installing the current version of IntelMausiEthernet.kext (2.4 as of this writing). While I was at it, I checked my other kexts and noted that FakeSMC.kext was also newer, so installed that (version 1800, and the associated sensor kexts) as well.
Finally forced by Adobe to upgrade to High Sierra (yes, I know, I'm on the bleeding edge!). Updates added to guide. The startup chime being the only problem I'm left with after the upgrade seems like good value to me.