Hello Nunya,
Yes, streaming from Spotify (or any of the supported music services) AS LONG AS YOU USE THEIR APPS, does not count against your 4G data limit (remember, after that limit you don't really lose data, you just go back to 2G speeds).
It depends. Bluetooth itself does not count as data usage, but whatever you use it for, might. If you stream Spotify to your car, then it doesn't because the Spotify app is the one using data, and in T-Mobile, that doesn't count. If you're using Bluetooth to stream the audio from a YouTube video, then the YouTube app is consuming data and it does count. If you use Bluetooth to stream a file in your phone, nothing is using data since the file is already on your phone.
I'm lost on this one. What do you mean by 'playing music from the lg g3'? If you mean that just using that particular model, then yeah, the same cases as above apply; it's the same with any phone.