PB blaster and make sure you have the right size spark plug tool, if the previous person didn't put anti-seize in it that could be why it's hard to break loose. to the expanding in heat, wouldn't the plugs also expand, i'm no scientist but they would do the same as the head would and expand correct.
pb is the shizz, been using it for stubborn bolts and girlfriends for years!!!

to strip it usually happens when you put them in wrong and cross thread the head, usually.