Isolate your experimentation, and work simple to hard, and cheap to expensive.
If you are certain the pump is moving to full throttle, this first step is done.
Glow plugs are easy to test, and you can attach a jumper wire to make sure they work. Even in warmer weather, my relabeled Beaver (A Satoh ST1440) won't start without the glow plugs.
I don't have a service manual for the Beaver, but my Yanmar service manual has instructions for setting the pump timing. I wouldn't use these numbers, but the procedures are probably very similar.
"Beginning if injection should occur at 23-25 degrees before top dead center....attach a tube to number one outlet of fuel pump (for Yanmars, this is the cylinder nearest the flywheel) then rotate engine in normal direction until fuel stops flowing out of the tube. This is the beginning of injection and should occur as mark on crankshaft pulley aligns with notch..."
It goes on to explain that adding shims will ****** the injection timing, and subtracting them will advance the timing of the injector pulse. I don't know if the Mitsubishi engine has similar timing notches or not, but it is worth a look. In rebuilding the pump it is possible the timing was altered enough so it won't quite start.
My Yanmar manual says the shims are 0.1mm thick, and alter timing 1 degree each. That is a tiny increment, and well within the realm of being altered by a rebuild in my estimation. Point being, if your glow plugs check out and it is moving fuel, you need to verify the timing is correct on the pump, too, then move to the injectors and compression testing.