For years I used a 5V BEC to power four 9 gram servos through a CC3D board. There was even some servo oscillation while I tuned the PID. Nothing ever went bad, but I never used 8.4V input power voltage, always about 5.2V.
It sounds like you think you have one good CC3D left. I would measure good CC3D resistance (lowest ohms scale with all CC3D cables disconnected) from PWM ground pins to power input ground pins, and again from PWM V+ pins to power input V+ pins. Both measurements should show a direct wire connection (about 0 ohms). That is a good reading. It will show you what a good board measures. Now measure a "broken" CC3D the same way. Did both boards measure 0 ohms? If so, then the traces on the bad board are OK and NOT the problem.
Important:
Another thing to know is that the CC3D PWM signal pins need a 3.3V signal but are "5V tolerant". If you have 8.4V power connected to CC3D and you plug in a servo, then
if (50%-50% chance) the signal pin (outer pin on servo connector) and V+ pin (center pin on servo connector) make contact before the ground pin (the other outer pin on the servo connector), then you have effectively connected 8.4V directly to the 3.3V (5v tolerant) PWM signal pin (through the servo which is acting like a resistor at this point).
You can see that the other chance (also 50%-50%) is that the V+ and ground connect first so it is not like just connecting signal to V+ through a resistor (the servo).
If the CC3D is actually broken, but the V+ traces are OK from power input to servo connector and the ground traces are OK from power input to servo connector, then this is what may have damaged the PWM signal output.