You should make your buddy appear here in person to take the heat for things like pulling the vacuum lines out without marking them... etc....
LOL
Not much excuse for those things once digital camera's got cheap....

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

It sounds like a clogged tank screen but, to be sure, I would switch the lines in the engine compartment where the hard lines turn to rubber. If it is clogged, the car should perform like it did when you bypassed the tank. The small filter should be fine if you just replaced it...unless it's black already.

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1981 300TD "The Green Lantern"
1980 300TD
1983 300D Euro "China Cat"

I don't think you can rule out a bad spin on filter. Probably not the problem, but the returning fuel - as you had things setup temporarily- wasn't going through the spin on filter. Return fuel just sort of flows past the top of the spin on filter to siphon off any air. If, somehow and however unlikely, the spin on filter is blocked or not sealing, you can't rule it out based on your test. Could still just be sucking in air at either the center bolt or the larger sealing ring on the spin on filter.

Unless you already saw a ton of gunk in the original plastic filter I doubt that crud in the fuel is the issue. It wouldn't have grown there in the time it took to adjust the valves .

Anyway, still worth checking the filter before you get into everything else.