Well ... today I test drove it all the way out of my neighborhood to the tire place three miles away, and got new tires (mine were almost 10 years old, on it since I bought it). I didn't see any leaks, so I took it to the DMV, about 20 miles away ... not only made it all the way there, but passed easily. Stopped by the MB dealer to pick up a part for my 300D, then drove all the way home in a downpour. Never felt so good being stuck in traffic in the rain. It's a world away from before, probably due to 1) the injectors and 2) fixing the lift pump, which was I guess the silver lining in my wild-goose chase.
There is some fuel on the injection pump and in a few crevasses, but I believe it is residual from the giant mess I made while bleeding it. Will clean it up and re-assess. But basically, the only glitch during my 50-mile drive this afternoon was when my knee jammed up while I was trying to work the clutch. Felt really good to drive it again; there were a lot of times last year I thought it would never go anywhere without being on a tow truck.
Thanks for all the help, guys, as usual.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

Congrats for sticking with it and getting it done! So, no more bubbles? Were you able to finally determine the cause of the bubbles?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

It's not easy to use a torque wrench on the line nuts. I tighten them (by feel) just enough so they do not leak. Take the line off and look under a magnifier at the flared ends. If you see a groove there that is what's making it leak. You may be able to fix it by polishing that groove away with 400 grit wet/dry paper in a twisting motion.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I have not seen any more bubbles anywhere! Compression gasses were getting into the line (and pump) via one of the remanufactured injectors. Once I removed that injector and put a spare in, it bled and ran. The reman set went back to Bosch and the dealer got me new remans under warranty. These work great ... breeze to bleed and it now sounds and feels better than it has since I owned it. My dad even remarked at how good it sounds now. I think I stopped the leaking from the lines, but I'll keep that in mind about the grooves if I detect any leakage with more driving.
Today I got it back from the alignment shop ... now I can drive it!!! Once the salt is gone, that is ... it's already got bad rust, but don't want to make it worse.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

So you isolated it down to one injector and it was combustion gas getting back into the IP? Wow, that's a first that I've heard about. More details please!

Frankly, I do not see how compression gas can go from an injector, through the delivery valve and into the IP. Maybe somehow the compression gas is going out the injector return barbs. But that is also unlikely, unless parts were left out of the injector when it was remanufactured.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I should clarify that I am not certain that is what happened, but it is simply my theory. I asked a few posts ago if anyone could corroborate that theory or disprove it and offer another one. Wonder if I should pull the topic into its own thread, as this one probably doesn't have as many eyes on it anymore?
My conclusion is only empirical, although I did have input from others. I could not do a necropsy on the suspect injector because it would have voided the warranty. When I took the next batch to a licensed Bosch diesel injection shop for precautionary testing and mentioned what happened and my theory, he agreed it was likely compression gas. I asked again for clarification that air can go backward through the injector, and he said if the pintle is damaged or stuck, or the spring is broken, it can happen. One of the mechanics at the MB dealer also said (this was relayed to me by the parts guy) "I've never seen it happen, but it can happen." Evan (KarTek) also helped me conclude an injector problem was possible after I eliminated supply-end leaks, though I should let him chime in himself.
To break down the empirical process that leads me to target the injector, touching only on the highlights and not all my melodrama that made this thread so long:
-The car ran, albeit lousily, before I installed the reman injector set. After installation it exhibited airlock and would not run. I assumed it was coincidence and that I had disturbed something else on the supply end.
-eliminated the possibility of leaks at the tank or near by using a bottle of diesel
-eliminated primary filter as problem by bypassing it. Put it back after that test.
-eliminated entry of air into the IP from supply side by replacing secondary-filter-to-IP line with a clear one. Never saw one bubble go in.
-realized car would run on three cylinders with No. 3 injector line cracked, and froth/bubbles coming out of that line. The other three lines would stay bled under that condition. If I tightened No. 3, it would run for a while, but eventually stall out, exhibiting air in the IP-to-secondary-filter line and air in all injector lines. But NO air moved through the secondary-filter-to-IP line at any point.
-switched No. 3 injector with No. 2 injector. Results were exactly the same, but NOW with the No. 2 spot the problem. I.e., the problem followed the injector. Everything else would bleed, but bubbles kept coming out of that cracked line indefinitely.
-purchased spare injector on eBay for experimental purposes, stuck it in in place of the suspect one. Started it with that nut loose, watched a few bubbles come out then sure enough, pure fuel squirting out. Tightened it and it ran like a normal engine. Idled for a while, took a short drive, kept running fine with no bubbles.
-sent the set of four remans back under warranty (the dealer recommended I send all four and get another four back). Got a new set of four remans, had them pop tested, installed them ... they bled easily, it started right up and ran fantastically.
To sum up: I changed and tested a lot of things, none of which affected the airlock other than the injector. Moving the injector moved the air. Replacing the injector eliminated it.
It makes sense to me that a defectively assembled injector or damaged nozzle could allow compressed air backward, though what confuses me is how it was getting into the IP. Evan pointed out to me that the delivery valve won't let anything go backward, so you would think the air would just compress in the line. But air was getting in the IP with that injector line tightened, and it wasn't getting in the IP with that injector line loose.

Man, that was a dissertation ... I feel like I'm back in college. I probably need to add some footnotes and references.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

On my (5 cyl) motor, I change 2 nozzles then start & run for 30 seconds or so, change 2 more, s&r, then the last one. Just a "few" extra seconds of cranking. On my 6.2 diesel blazer, I go 3, 3, & then 2. Works for me. -corne-

note - FWIW the wd-40 uses propane as propellant, this is what aids starting. If you dare, try using a propane torch (unlit, duh). I used to run several excavators which had 'deetroit diesels' for power. The all had cold climate starting aid units on 'em. They used standard disposable propane tanks. I've done my Blazer w/ the propane torch, never needed to with my MB.