I have been through this firedrill more than once. All electrical measurements are okay, but if you will pull a few glow plugs, I'll bet you find at least one with a hole burned in the outer shell, exposing the heating element inside.
I know this doesn't seem to make sense, because it would seem that only one would cause problems in one cylinder. But my experience has been, replace 'em all and the problem is fixed. Let us know if that corrects your problem.
You are correct about the parrallel circuit, however, if you unplug the connector inside the top of the glow plug relay, the glow plugs are isolated and can be checked individually with an ohmmeter. As a matter of fact, you can check them all from the plug you disconnect at the relay much easier than probing underneath the fuel injection lines.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in