Will 5x get the music player update of 5x plus?

I believe it is lacking any bluetooth transmission other than BLE. So you'd have no way to pair headphones to the watch.

That's correct. It's missing the proper BT hardware. This was asked many times back when the fr 645 Music came out a few months back.

Interesting article that I found in Business Insider after a quick Google for BLE Audio

"Bluetooth audio is set for a notable shift, and it could help usher in a new generation of headphones. The ubiquitous wireless standard will move all audio applications onto a new, low-energy radio, called Bluetooth low energy (BLE), by the end of next year, according to Mark Powell, the executive director of the Bluetooth Special Interest Group (SIG)".

Maybe it's not as unlikely as we may currently think.