The heat line probably goes about 10 ft town the well. It doesn't NEED any more than that.
You could probably cut the pipe in the vicinity of the crack and pull back the end from the well; but no guarantees. It is hard to push a wire around an elbow. Try pulling it back a few inches and then try to push it back in. If you can't push it a few inches, then you will be unable to push it several feet. It was probably pulled in the first time.
You might try pulling the end back from the house to the break if it will fit through the pipe, with a wire to pull it back through when you are finished. You might have to cut the plug off and splice it back on. You will have to cut and splice your pull wire after you add the repair section.
You can make up one end of the removed section with a coupling; then install the union on the ends of the pipe.
Then you can push the free end down the well, if you are lucky, and make up the union.
It is probably possible to cut and splice the wire, using a splice such as is used for connecting the pump under water.
By the way, a heating wire is not a pyrotechnic. Pyrotechnic refers to fire, such as fireworks or explosives. I didn't understand what you were referring to and my response didn't consider that.