Regarding your questions about BT802:
1. BT802 is SoC module, it can work without an external MCU.
2. The button functions is not configurable, if you need to change the functions, it would need customization of firmware
3. BT802 would open SCO channel to transfer audio over HFP and open ACL channel to transfer audio over A2DP
4. Latency. If you buy aptX-LL license from Qualcom, the audio latency will be about 40ms, otherwise, the latency will be tens of milliseconds higher
5. You can pair more than 2 BT802 devices, play music over A2DP simultaneously, but it can't talk over HFP simultaneously.