That is a bummer but it sounds like you are right, head replacement (or at least repair off the engine) is the only good option.
I think you are also right that the problems with the injector threads were not your fault, they were probably caused by the owner before you. If the injector is not sealed fully into the head (i.e. if the sealing heatshield is not replaced and/or if the injector is cross-threaded during installation and does not meet the seal flush), then just like you describe, the leaking compression will cause the threads to become packed with soot. Then it becomes very hard to unscrew the injector and could lead to destroying the head in the attempt.
I actually have a tool that someone made from the bottom half of an injector with slots cut into the threaded part, intended to be used as a thread chaser to clean up the threads in the cylinder head after soot leakage has occurred in a situation like this. But if the injector cannot be removed without damage to the head casting itself, like in your case, it doesn't help.
Good luck and hopefully at least it gives you a good opportunity to refresh some other stuff while you are in there like timing belts, etc.
