1, Yes spotcast.start does appear in the dev tool list
2, ‘Master Bedroom Speaker’ was lifted straight from the devices_json:
3, it is correct because the sensor it populated with the list of devices
4, yes I do have a premium account
using the above I posted as a script I get the following error
Invalid config for [script]: expected dict for dictionary value @ data['script']['play_song']['sequence'][0]['data']. Got None
extra keys not allowed @ data['script']['play_song']['sequence'][0]['device_name']. Got 'Master Bedroom Speaker'
extra keys not allowed @ data['script']['play_song']['sequence'][0]['uri']. Got 'spotify:track:6xFXusME7FMMyMkPrIv2WJ'. (See /config/configuration.yaml, line 176). Please check the docs at https://home-assistant.io/components/script/