I don't know about the headlight. Quite possibly it doesn't match your controller wiring in any way. The battery indicator would of course need a + and - wire to work. Presumably the light itself works on your 36v pack voltage as well, so I can't guess what the third wire does. Maybe popping it apart if it can be, will at least give you a clue which wires go to which part.
Brushless motors have three phase wires, as you likely know. Then a sensored one has five more thin wires to hall sensors in the stator. Sensorless runs without the halls to help the motor stay in synch, so a sensorless motor will have no wires or halls. A motor with sensors can run on a sensorless controller, just don't connect the halls. Some controllers now come so they can work either way. If a hall sensor stops working, the motor still runs in sensorless mode.
Among the combinations of phase orders possible, one should run good forward, and another backwards. Swapping two can reverse the motor. Particularly easy to reverse a sensorless motor, since there is no need to also swap halls wires.
Ebrakes have a reed switch in the handles, so that when you pull a brake lever, the controller is powered off. I tend to consider them pointless myself. But on bikes with brushed motors, they have a good reason to be. A brushed controller often fails in the full thottle position.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.