RE: Too much voltage?
Unfortunately, it's not a simple yes/no answer. There are a lot of factors at work here. The best anyone can say is it MIGHT work with an extra cell, especially if you're not using it to its full Amps capacity. Personally, I wouldn't chance it, but that's me.
Yes, 12 Volts will fry the receiver. In fact, anything above ~6.5 Volts will fry the receiver! That's why most ESCs have battery eliminator circuits, BECs. A BEC is a voltage regulator that will reduce the voltage from whatever the main battery is down to 5 Volts. Most built-in BECs are good for batteries up to 10 cells (12 Volts). After that, you need to get a separate receive pack or an Ultimate BEC.