Well, it sucks that it is getting hard to fix. But I still think it ought to be something simple. You say it turns over with smoke, that suggests to me no pre heat.

What do you need to make a diesel go?

Fuel and compression, and in a precombustion engine , pre-heat.

You say you have fuel at the injector lines ,
320 and 410 compressions, so it should light up on at least some of them.
Need pre heat.

If I were there , my next move would be to pull the glow plugs and hot link them individually to the battery to visually check they glow. Then clear the carbon from the hole they go in. The 617 starts pretty good with glow and compression ok, the only other thing that usually gets me is an old battery running out of grunt.

Still no go? Is it water in the fuel? Is the engine cranking fast enough?

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'72 240D
'76 300D
'88 300sel
'40 Chev pickup , the shop hack.
'73 Monte Carlo Landau, for "Super Chevy Sunday"