Thanks everyone for all your advice and suggestions. A few followups:

The stopcock is a MB part. I suspect that only the earliest w115s got them. They were more typical of the prior generation of cars (w110s I vaguely recall). Or perhaps the original owner moved one over from one of those cars.

I was only using distilled water for what I hoped to be my last flushes. I wanted to do one entire purge with distilled, so that my final fill was adding to what was hopefully mostly distilled water hiding in the block/heater.

I think I will probably just finish the job and move onto the next battle.

Regarding anticorrosion, what is the lowest percentage antifreeze I could get away with if I was willing to do annual coolant changes? I want to maximize the heat removal but not contribute further to the internal rusting of my engine. The lowest temperature ever recorded on San Luis Obispo is 12F (-11C), which corresponds to a 25% antifreeze mixture. That is well below Zerex's lowest recommendation for G-05 (40%).

My car used to reach 250F in 70F weather heading up a local freeway hill (I would pull over at 250F). After all the flushing efforts, with pure DI in the system, it "only" reached 230F which is way better. If I kept the heater on the other day, I was able to keep it at 212F going up the same hill. So the the citric acid and flushing has definitely made an improvement, which I am excited about. But if I go 50/50 coolant/water, the 230F I witnessed could end up being much higher, particularly if it was a warm day.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).