tl;dr -- if you use Windscribe, update to v1.70 prior to installing 10.13. If you used to use Windscribe, and lose all network connectivity after your High Sierra installation, delete the com.aaa.windscribe.* files in /Library/LaunchDaemons.
Long story short, the issue was Windscribe v1.61 Build 9 -- I had tested WindScribe (VPN provider) at one point, but it wasn't active. The leftover /Library/LaunchDaemons/com.aaa.windscribe.firewall_on.plist file was the specific culprit (the Windscribe firewall is intended to disable internet connectivity when not connected to their network, but wasn't in use on that laptop prior to High Sierra installation -- not sure why the plist file had been left behind).