Many airplanes (not sure about the CRJ) also have a center wing tank, which is directly under the fuselage, so would be kinda hard to separate cleanly. Also, explosive bolts might start a fire that wouldn't have started without the bolts...
And they purge the fuel, if its a controlled emergency landing.. but in a uncontrolled emergency.. to let go of the wings seems a good option for events to go.
No, they don't. The CRJ, and in fact most smaller airliners including the A320 and 737 do not have the ability to dump fuel.
In larger aircraft that do have the ability to dump fuel, the reason is to make the airplane lighter so it does not have do an overweight landing and the subsequent high-cost overweight landing inspection. It is not done to reduce the probability of a post-crash fire.
