A non working XL5 can be caused by numerous issues, one being that traxxas makes excellent plastics, but electronics not so much...
Servo power without throttle could simply mean it does not arm because your throttle signal is bad, e.g. the ESC lost the calibration, you fiddled with the trim, expo, dualrate or have a problem with the trigger. Try to recalibrate Throttle signals as stated in the manual.
Then, the Rx is known to die instantly when you plug the ESCs servo plug in there in reverse and switch on. Disconnect the motor (!) and swap Ch1 and Ch2 plugs on the receiver, then power the model. You should now be able to steer with the trigger. If you can't you've blown the Rx.
And: did the XL5 get wet? Water and Dirt cause corrosion, which may cause the XL5 to falsely think it is still rolling, usually you notice this when it does not want to reverse. The three coolers are different potentials and high ohmic connections between them cause this. Clean it if dirty.
It signals this with the LED. Your's is not coming on at all, i had this with my Stampede in the snow: Coolers were wet and suddenly only servos work. As it was dark we stopped playing, and when i looked the next day everything worked again, only to stop again after like 20 minutes fun. After cleaning the car with compressed air it did work again and then it dawned on me: adding a few drops of water on the XL5 and the light went crazy.
As for the soldering heat: I've never seen anything damaged because of a soldering inches away on a thick copper wire. Heat travels along the copper, and maybe it's too hot to touch, but electronics has no problem with prolonged exposure to 100+°C, and your fingers start burning at 60°C. Soldering is done with 250 to 400 °C and does not damage parts for like 5 seconds or even more, so if you're not excessively trying to melt anything and use a reasonable electronics soldering iron nothing should happen.
You melt the plastics, Deans plugs are notoriously known for that, but as long as neither Epoxy nor casing of the ESC are showing signs of excessive heat you're good at this end.
And the last thing: Do you have any other wiring in your car except Battery to ESC, ESC to Rx, RX to Servos? Anything that connects to the battery except the ESC may cause a loop that bypasses BEC or switch signals.