I could see 1 or maybe 2 injectors being worn and sticking shut, but not all of them at the same time.
If you are questioning fuel quality, I always say trust your instincts- drain it and replace it. You can save ot in jerry cans and add back in 50/50 once truck is running if you want. Or hook up a jerry can feeding the lift pump. Just remeber you have to cycle the engine enough to replace the old fuel with new when trying to start.
Where would I start?
Pull the glow plugs real quick (ok 2 arent quick so skip them 2) and crank it over. You should have a fuel mist from the 6 you do pull.
Then a quick bench test on the glowplugs that are already out.
5 minutes to do that and you know that you have proper fueling and working glows. Want to spend another 20 minutes and really know where the engine life stands, pull the all 8 glowplugs before "mist test" and then do a compression test.