Soaking with penetrating fluid and working it back and forth like you have been is the best way I can think of.
I've heard a 50/50 mix of brake and AT fluid works good.
I've personally never seen a plug break off but have seen them take the threads out of the head. Doesn't mean the head is garbage you can just repair the plug hole. Should take the head off if doing more then chasing threads so metal shavings won't end up in the motor. Coat the threads of the plugs with anti-seize when you install them.

1990 LWB RS A/T
1990 LWB Base A/T
1991 LWB LS A/T Going...Going...