If basic troubleshooting fails, what advanced steps can I take to fix 'Moonlight connection refused error 1'?

If basic steps don't work, consider a clean reinstallation of GeForce Experience and NVIDIA graphics drivers (using DDU - Display Driver Uninstaller - in safe mode can ensure a clean slate). You might also try reinstalling the Moonlight client. Check the Windows Event Viewer on the host PC (under 'Windows Logs' -> 'Application' and 'System') for any errors related to NVIDIA services or networking around the time the connection was attempted, as this can provide specific clues. Temporarily connecting the host and client via an Ethernet cable can rule out Wi-Fi interference.