WD40 wont hurt a carbon frame and you should be able to apply it precisely to the crank threads.
To remove a stuck pedal you need a lot of torque so use the longest wrench you can find or fit an extension. Bike shops usually have a special one for this purpose.
I use carefully applied foot power thus:
Put the bike in lowest gear and aply brakes.
Point the relevant crank foward and up.
Place the wrench on the pedal pointing rear and up.
Place foot on end of wrench and take up the tension.
Carefully apply more force until all your weight is on the wrench. Beware of the wrench slipping.
To prevent sticking, smear grease over the threads.