This is the kind of situation that reading the glow plugs output voltage properly should give some answers about the individual cylinder contribution to the overall running of the engine or power balance. You need the firing order in sequential balance for the engine to run in a reasonable manner.
Might even prove the pump is bad by indicating extreme differences in individual cylinder timing from the pump. It displays this by indication of temperatures present in the cylinders while runnng.
You should read up on the subject and get a good grasp of what you are doing. It is possible someone in the past adjusted components on the pump they should not have touched. Or perhaps general aging is involved. If you are really lucky they only messed up #1 injection timing. But if someone retimed the pump later it could be way off for the other four cylinders for example.
The milli volt method will clue you in if properly used. There is a danger that you have one cylinder running too hot already and that can damage a piston at some point. You know your injectors are reasonable and compression seems pretty good so should be worth the attempt. Put another way you can do this yourself and have nothing to loose.
I and others can guide you along the path on site if you desire. It is what I would do rather than throw money at it and just continue to guess what might be wrong. I cannot see your cam to see if it is too far gone or not. Sure it could be the problem but if so the mili volt method will even indicate where the effect of the problem is taking place. Opinions are just opinions. Proof of fault is better.