My 96 did not start 2 weeks ago, then it finally did after what I assume was the starter pushing the air out of a line. I have been waiting for it to do it again before I take the intake off and dig deeper. Plus my shop manual has not arrived in the mail yet. (I do not want to get into that if it was just dirt in the fuel)
All the fuel lines and seals have been replaced on mine within 10k so I am still waiting but its starting great so far. I hope you do find what it is and please post when you do.
Most every post that I have read has been a fuel leak from one of the injector line seals when having starting issues.
Man having that many people look at it with the problem still going on he has to be pulling out his hair.

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1999 E300 TD 190,000
1996 Passat TDI 225,000-sold
1996 E300 120,000-sold
1998 Jetta TDI 186,000 -sold
1983 Chev Suburban C20 6.2-sold