they're comparing magnitudes of pain that are too close to each other.

Doesn't that make the argument stronger? I mean, if you're not even sure that 13 months of torture are much worse than 12 months of torture, then you should be pretty confident that 10^6 instances of 12 months' torture are worse than 1 instance of 13 months' torture, no?

Such a quantitative scale would necessarily include several qualitative transitions

So that was the option I described as "abandon continuity". I was going to ask you to be more specific about where those qualitative transitions happen, but if I'm understanding you correctly I think your answer would be to say that the very question is misguided because there's something ineffable about the experience of pain that makes it inappropriate to try to be quantitative about it, or something along those lines. So I'll ask a different question: What do those qualitative transitions look like? What sort of difference is it that can occur between what look like two very, very closely spaced gradations of suffering, but that is so huge in its significance that it's better for a billion people to suffer the less severe evil than for one person to suffer the more severe?

(You mention one possible example in passing: the transition from "PTSD" to "total lunacy". But surely in practice this transition isn't instantaneous. There are degrees of psychological screwed-up-ness in between "PTSD" and "total lunacy", and there are degrees of probability of a given outcome, and what happens as you increase the amount of suffering is that the probabilities shift incrementally from each outcome to slightly worse ones; when the suffering is very slight and brief, the really bad outcomes are very unlikely; when it's very severe and extended, the really bad outcomes are very likely. So is there, e.g., a quantitative leap in badness when the probability of being badly enough messed-up to commit suicide goes from 1% to 1.01%, or something?)

a moral theory that leads to not being able to choose between 2 persons being tortured for 25 years and 1 person being tortured for 50 years

If you mean that anyone here is assuming some kind of moral calculus where suffering is denominated in torture-years and is straightforwardly additive across people, I think that's plainly wrong. On the other hand, if you mean that it should be absolutely obvious which of those two outcomes is worse ... well, I'm not convinced, and I don't think that's because I have a perverted moral system, because it seems to me it's not altogether obvious on any moral system and I don't see why it should be.

a decision theory that leads to scenarios where small questions can quickly escalate to blackmailing and torture

I'm not sure what you mean. Could you elaborate?