So what's the state of the art for testing PWAs using websockets on iOS devices on a local network these days, preferrably something with automated certificate generation (assuming wss is required)? Or should I still proxy everything from a live public server (with valid certs)? 

 Depending on how much "hardware" you actually need to test, the iOS simulator running on the same machine is probably the easiest. 

 Depending on how much "hardware" you actually need to test, the iOS simulator running on the same machine is probably still the easiest.