Thanks Cato. I uninstalled NM as suggested and re-installed it from scratch, and it’s working well now so I don’t know what had happened to the original installation. There are no SAMPLE files in the etc. folder in the new installation so I’ve no idea where the ones in the original install came from.
I’ve just been playing Edith Finch from my PC (Windows 7) to my laptop (Mint Cinnamon) over my LAN and it’s perfectly playable – graphics, audio and mouse response. If it’s as good with the old Myst games I want to work my way through again soon I shall be quite content.