sounds like it was stripped when it was installed. btw, i've never heard of applying anti-seize to plugs as the plug has to be grounded to the case and anti-seize would only get in the way. then again, i could be wrong about that. one thing that you MIGHT try is to work the plug up to the sticking point and back down, then out again, repeatedly. backing down gives the plug threads a chance to break the chip. i've done this before but never on anything nearly as valuable as a case. an other suggestion as to what to do about #3 though is to back it out and re-tap the threads. the bad part is that would leave metal debris you'd have to remove from the cylinder. you've got an issue on your hands my friend.
