Got a reply from GOG Support, however it was just the stock "please try these things" and nothing specific. Update graphics and sound drivers for example (why would sound drivers stop my mouse pointer appearing?). Did indeed do them all and nothing.
We have those "please try these things" in place so that we can eliminate common causes to technical issues with games. Once the common solutions are shown to not work we are able to know better that a given problem requires more work.