Yah, I know, I should know all about this, but I live under my sunny rock.

With cold weather, the ability of the Lead Acid flooded cell battery to
deliver starting current to the vanagon starter is diminished.  Yup.

How low can the voltage go before the ECU decides to not produce FI
pulses?  9V? 8V? or?

The internal resistance of the battery is a function of temperature and
with the ideal battery that resistance is zero. ( No, Ben, Ideal batteries
are not available so we have to make do with what is available.  )  As
batteries age, the internal resistance rises, much like mineself.

IFF you measure the voltage at the ECU under starting loads what should you
read.  IF you measure the voltage at the battery terminals under the
starting loads what should you read?

What are the boundary conditions for these voltages and what is the no-go
threshold?  Surely one of the exalted members has done extensive research
in this matter and would be pleased to offer data.  (We already have lots
of opinions, mostly divergent) .

I no longer own a WBX nor it's FI system so I can't do the experiment.
FWIW establishing these boundary conditions is neither simple nor easy.

Pensionerd.