Yea you can run Music Assistant without HA but then you might as well get the home automation benefits.

I’m not exactly sure what you are asking, it sounds like you want to be able to play music to a speaker or group of speakers without being in the same location, but then you wouldn’t be able to hear it. I think maybe Sonos stuff can be controlled like that but I am not sure. Otherwise why would you need the device itself to connect to the Internet and not the device initiating the casting? (Though in some cases they, especially the chromecast, will hand the stream to the casted to device rather than streaming it from the initiating device across your local network but I don’t think that’s what you are asking about either.) AirPlay for instance will just play whatever audio is currently playing on the casting device no different than if they were headphones. You can also set up AirPlay receiving on things like a Raspberry Pi if you want to try it out before buying dedicated hardware.