lol, I'm hardly an expert, but what Arct1k said is what I'd do, in the following order:
1: Send it in. I know that's not an option in your case, but I gotta say it 'ya know?
2: Try to find another spot to solder. You have to be careful if you plan to use the fan lead because that power is turned off and on based on temperature. However, Castle may be switching the ground lead (meaning the power line is always "hot") so you might be ok. The only way to test would be to power up the ESC and
carefully use a multimeter and measure. Careful not to accidentally short stuff out with the probe tip though. Also, be aware that when poking around, you might find a spot where you see 6v, but also measure AC (with a true RMS meter) to make sure you aren't measuring a sqaure wave that happens to average out to ~6v DC.
3: Keep the red wire out and use an external BEC. Be aware that programming will be a PITA though. You'll have to hook your main battery pack while the castle link is used so that the brain board gets power (normally gets it via the link but since the red wire is gone...). I know this works because I'm using an external BEC and that's what I have to do to program my MMM.