Well, the moral of this story is going to be to always grease the threads, and whenever a cap fails to yield to a normal amount of loosening torque, to apply penetrant and impact the cap's periphery with a sturdy dowel before using any additional amount of torque.
All of which has never failed me (other than the times that I didn't do this of course).

And as for "wiggling in" a tool bit or "gently" hammering on it, I usually go big or go home. It's very hard to do any damage hitting the head of a crank bolt, so I aim for a complete bottoming out of the hardened bit in the hole. The hammering and resultant wedging action serves to cold-work the bore into a cold-forged net shape that matches the exact shape of the tool.
And again, with vigorous blows it is most advisable to hold the bit with a Vise-Grip and to wear safety glasses.