I was also able to use the stock Settings.apk app that many others have had success with when adding Bluetooth devices.
I figured since most of the app is not useful with the FireTV, and some of it could be dangerous to mess with - I updated the APK so that only the Bluetooth settings are available, nothing else. I also updated the app title to Bluetooth Settings (English only), and used the stock Bluetooth icon as the app default. So now instead of seeing two Settings apps, this one is better differentiated.
- This will not fix any existing Bluetooth issues, this is the Settings.apk app but without the non-Bluetooth menu items
- I've used this on a FireTV Stick to pair a Favi FE02BTBL touchpad/keyboard combo that needs the keyboard to enter a random code for pairing
- The Bluetooth ON/OFF switch button was not removed, so you still need to be careful not to turn off Bluetooth
The only thing this app offers me as a setting is to turn of BT. Nothing else. I installed it immediately.