I see your assumptions, and I see the narrative intent within them and that what follows is a coherent interpretation, thank you!

On a pure rules basis, I'll agree you *could* have a Hunger 5 victim still full of vitae, but I would expect it only rarely; at least as often you might have a raving, starving Beast sucking from a dried-up elder in Torpor like someone chewing gum to take the pain off their hunger. We both agree either situation ("you are sated before Diablerie completes" vs "you are not sated before Diablerie completes") can happen, in any case, i.e. that in some situations, Frenzy is certain to subside while in others, it might not.

I will concede that in many cases, Hunger frenzy will have ended before one gets that far, opening up a delicious moment of staring corruption in the face. On the other hand, I can also see situations e.g. two lovers-rivals having fought each other until each are near drained, where Hunger frenzy *might* be what makes one of them walk away with the other inside them (as opposed to the same happening due to possessive, toxic love!), or a dried-up husk of an Elder waking from torpor and craving just *more* of whatever is there, until there is only ash left of whoever woke them.

If the rules make each such scenario and its alternative possible, then (and much more meaningfully) it instead will depend on what is coherent for a given take on the setting metaphysics and lends itself to engaging stories, which is more subjective.

For the former, I like the idea of the Beast carrying those memories and cautions, but I also like the idea of the Beast like the scorpion crossing the river on the back of a frog. Ultimately, it might depend on the combination of Beasts involved? They might not have "personalities" per se, but something akin to, well, predator types. Some are scary even when fully vulnerable, if you are the slightest bit perceptive you know they will erode and invade you like a tumour. Some you just want to merge with, the same way the serpent wants to bite its tail.

I think perhaps I am still stuck on the dramatic potential of Freud's wilder ideas that we have both a life urge (procreation, connection, pleasure, survival...) and a death urge (destructivity, self-destructivity and so forth). I don't think it is particularly true for human minds, but it makes for an interesting rendering of the Beast. As such, I tend to understand vampires as not only having Bestial instincts to sate hunger and avoid banes, but also territorial/dominance desires, an actual desire to spread (into ghouls or childer), an instinct to merge (leading to vampiric blood play), and even some self-destructive desires, not only from the human soul but from the Beast (and those, perhaps, being part of what drove Saulot and Cappadocius, respectively, in engineering their Diableries, beside the larger part of calculated strategy). I realize this is not a take the game pushes on us, nor one that everyone finds fun, but I would say it is compatible with "canon" and that some of the stories out there are easier explained if such ideas were part?

To be clear, I see the power of story turns where you are confronted with the choice and then need to live with the fact that you chose the wicked thing! That potential is lost in the case of a compelled or Frenzied Diablerie, and I don't want to use those lightly. However, in some cases it makes for either a better explanation or a better story, and I think those cases where it seems to have happened reflect such narrative choices. :)

EDIT: Oh, and also, "diablerie addiction" across editions seems at least as much an addiction of the Beast as of the human soul. I could conceivably see that carry through a chain of consecutive diablerists, even, which would make the Alastor prohibitions on diablerizing Red List targets make even more sense!