I think I have it licked.
Since I posted last I replaced the battery in a moment of weakness grasping at straws. That was dumb. I wish I had at least kept my old battery for a computer UPS backup project instead of turning it in for the core.

All the symptoms were just so consistent with a glow plug problem so I went back rooting into that system and started re-testing stuff. I realized that there was one plug that I did not pull to test externally. The second one from the rear.
Sure enough it was dead.

I replaced it and it started easily and ran quite smooth immediately.
Last night it was 26f and this morning it started without hesitation.

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84 300SD 274K
38K miles on flatplate heat exchanger and various diesel/veg blends. prior to that 4K miles on unheated veggie blends with kero and DinoD.