Ok, found out I had to enable both wi-fi & ethernet at the same time. Strange though, picture is VERY odd - kind of "sparkly" - don't think it is suitable for watching anything. Anyone have alternatives for screen mirroring? Thanks!
Quick tip on the Miracast side: even when DXDIAG says your PC supports it, the wireless adapter has to specifically support Wi-Fi Direct virtual adapters and Miracast sink/source. Lots of older Intel and Realtek chipsets report Miracast as available but fail to actually negotiate. A cheap USB adapter with explicit Wi-Fi Direct support usually fixes it. And yes you can keep ethernet for normal traffic and use the wireless adapter purely for the Miracast connection. Set the ethernet as your primary in the adapter priority list and it will route normal traffic through that.