Somewhere to start: If a service like media_player.turn_on (or light.turn_on, etc) doesn’t receive a payload (the entity_id), it’ll turn on all.
So it appears that the body is not making its way to HA. I’m not 100% sure where you can see what HA’s receiving, though. I suspect you’ll need to set the logging to a higher level (like to debug) to make sure that the dasher side of things is working correctly. It appears that you’ve configured “body” correctly.
I got the Dasher Component working, I had to remove “debug” true. I was using a REST client to send the invocation and for some reason it isn’t reading the body. Must have something to do with my configuration on the Client.