I just want to point something out that may, or may not have anything to do with what you're fighting.
The ignition switch on these cars can be flaky sometimes. The one on my daughters '81 300D had to be turned back to the left slightly after starting in order for the climate control system to get a feed. I just turned a little to the left and everything would work fine. I since replaced the ignition switch, and no more problem.
Since you appear to be possibly fighting an intermittent problem, try this and see if it has any effect. It could be that sometimes the ignition switch is making contact, and other times not. This would definitely set up a variable in your troubleshooting that could drive you as crazy as it did me.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in