If it ran for 20 seconds, I would think it should start again unless something catastrophic happened. Hold the accelerator half way & turn the fast idle control on the dash all the way to the left before starting and don't turn it down until the engine has run awhile and the temperature gets above 120 degrees. With my car I've never found throttle position to have any impact whatsoever on cranking time, but when cold, more fuel is definitely needed to keep it from quitting as soon as it starts to fire.

Has the battery suffered some deep discharges from all the start attempts? If so you might have to try a new battery entirely at least until you've got it starting easily again. I've noticed that connecting a good battery is often more successful than jumping or parallelling a bad battery with a good battery.

Yes, the glow plug indicator goes out while cranking to prevent additional voltage drop, but the glow plugs still glow.

BTW when I replaced my starter, my friend and I removed the air cleaner assembly and lifted the starter out from above. Having two people helps a lot....one to help guide it from underneath...that thing was heavy!

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1976 240D "Katja"