"However, at the current firmware, if the buttons are locked during screen off, then it also won’t be able to control the playback via the Bluetooth headphones."

This sounds like you are saying that locking the buttons on the device also locks out any external controls from the headphones over AVRCP?

Yes. If you lock the buttons device, then you can;t pause/play or skip tracks from your headphones controls

Incorrect, I can enable the button lock and still control all aspects via my BT headphones just fine. I haven't tried with wired headphones as yet. This is on a release unit running FW 1.2.

It may have been updated with the 1.2. I used it with both 1.0 and 1.1, and had the same results