re:  When starting up the engine up cold, with the temperature below
30 F, the engine misses and barely runs for the first minute or so. I
have to give it throttle just to keep it running.

Not specifically familiar with waterboxers, but most Bosch fuel
injection systems have "cold start injector," another (5th) injector
that only operates at cold temp to provide "choke" effect.  Since
there's only one, it's not close to a particular cylinder but rather
upstream in the intake manifold.

I find it interesting to relate just why cold engines need "choking"
in the first place.  That's because the fuel portion of the fuel/air
mixture tends to come out of the mixture and adhere to cold manifold
walls, not making it to the combustion chamber.  The solution has
usually been "choke," i.e. making the mixture richer so some of it, at
least, gets there.  Reason some intake manifolds are heated, too.
Choking is usually accompanied by increased throttle opening to get
things moving.  This "choke" period is a period of high exhaust
emissions, and the goal is to end it as soon as possible.  Probably
the next generation of gasoline engines will feature fuel injection
directly into the combustion chamber (like diesels and '50s Mercedes
gullwings), and this could eliminate or certainly shorten the period
for choking.

Operation of this and any fuel injector is a difficult and dangerous
thing for an amateur to test.  Since you say you have to give it
throttle, perhaps your choking is too much (worn/enlarged injector
orifice?) or the programmed rpm increase isn't happening.  Cold start
is easier with synthetic motor oil too.