I had a TD motor that had the same issue with the injectors. One heat-shield was completely missing, and 2 of the injectors were so badly coked, that I really had to work them back and forth to get them out. In the end, I had to chase the threads with a bottom-tap to clean up the holes, and one was still a bit less-tahn-ideal, though the injectors still threaded and torqued in there oK. My guess is that your low compression on some cylinders is likely due to carbon buildup on the rings. If you let those cylinders (and or all of them) soak in a bath of kerosene for about 2-3 weeks (keep topping it off, because it will bleed into the pan). Then, after three weeks, blow it out by cranking the car with the injectors out. Finally, drain the oil and put in fresh oil and filter. I'm willing to bet your compression numbers come up the "drivable" range.