Nice mapping!
You found all four transistors - each of them switches one color of two LEDs. So we have 2 pairs of LEDS, and two colors - therefore 4 transistors.
So (as seen on rear LEDs) red goes (from transistor) through 400 Ohm resistance, and green through 370 Ohm? Looks like this is used to make same brightness of red and green LEDs.
I would assume the circuit for front LEDs should be identical to the one for back. So there should be one more 200 Ohm resistor before M1_R, and two more before M2_R.
Also, 370 Ohm resistors linking M1_G and M2_G to their transistor. When transistor is disabled, you should see VCC on output pin of the transistor. When transistor is enabled, that pin should be shorted to 0V (in theory it could also be shorted to VBATT, but I doubt Dji does such bad things).
To test whether a transistor is switching correctly, just short the M12_{RG} or M34_{RG} to 3.3V output or GND for a second, and check whether this causes the diode to light/disable. M12_* and M34_* are probably going through high resistance low power resistors before reaching the transistor base pin; so if you want to be safe, short the transistor base pin instead of M12/34 lines.
The +VBATT you have on the back does not sound correct. I would assume there should be only GND and 12V used around the transistor. But who knows.
EDIT: Wait, there is a schematic available on the project you inserted picture from:
So you have a schematic; then what are you missing?