The lift pumps move a LOT of diesel. If the engine is rotating, the pump is pumping. You could have air ingress, hook up a piece of clear tubing in place of your return fuel line and see if you have bubbles in it. A very common leak is the fuel preheater thermostat.

Unless you dumped in several gallons of 2-stroke oil, there's not any real thickening of the fuel.

How did you test your injectors? Do you have a pop tester? Did you check pattern and leakdown in addition to just the pop pressure?