How can this (perhaps narrow) region of the HAZ not be nearly-
>or fully-hardened, and therefore more brittle? Is there some-
>thing about the fact that the excursion through the critical
>zone and back was so quick that limits the amount of hardening?
Slow (enough) cooling is one good way to prevent the "quenching" that brings on high hardness/martensite/brittleness. I think that is behind the common modern recommendation for Preheat in cold weather or when welding thick sections.