hot hell, cold hell, hungry ghost realm, etcetera: traditionally, are these seen as useful allegories/fictions for the monks to visualize, or external places w an objective existence that one "visits" during the meditation?

This is probably the best overview of realms from a practical meditation perspective: Hot blooded kindness (arobuddhism.org)

There is also a very good discussion of realms in "Wake Up To Your Life" by Ken McLeod

Interesting explanation, in particular that Devas Realms is not only about Impermanence but also about why Oneness (Non-Dual) is an (un)holy trap. I tried to summarize the 6 Realms in "in a nutshell" way, but there's always different angles to consider, so not satisfying. Anyway, here's one possible view:

Hell       ->   Hostility Patterns (violence)
Ghosts  ->   Addictive Patterns (dependence)
Animals ->  Reactive Patterns (unconscious triggers)
Human  ->  Desire Patterns (wishes, expectations)
Asuras   ->  Rationalization Patterns (mental striving)
Devas    ->  Prestige Patterns (status quo bias​​​​​​​)

Thank you, reading Ken McLeod now. Seems to be what I've been looking for, especially his explanations of karma/patterns. I've always struggled seeing the point of pursuing attainments or enlightenment or what have you, but I love this explanation of dismantling patterns because this seems directly useful - to the extent one can do it, one obtains increased freedom right here, right now, regardless of what other attainments one achieves or not. Further, it seems to be patterns are at the core of suffering. If attainments result, cool. But the work on dismantling patterns seems meaningful in and of itself.

Yeah, the KMcL is one of the few people that talks about the tibetian stuff in a practical way in very clear english.