Just because you have 12 V dc on the glow plug terminal does not mean the plug is glowing...

Cracking the injector nuts is needed to get the air out of the high pressure side of the system. If you have air in the hard lines, it will be trapped between the delivery valve and the injector nozzle. The trapped air acts like a cushion and the injector nozzle can't pop until you let the air out.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech