If dust specks have a value of 0, then what's the smallest amount of discomfort that has a nonzero value instead?
I don't know exactly where I'd make the qualitative jump from the "discomfort" scale to the "pain" scale. There are so many different kinds of unpleasant stimuli, and it's difficult to compare them. For electric shock, say, there's probably a particular curve of voltage, amperage and duration below which the shock would qualify as discomfort, with a zero value on the pain scale, and above which it becomes pain (I'll even go so far as to say that for short periods of contact, the voltage and amperage values lies between those of a violet wand and those of a stun gun). For localized heat, I think it would have to be at least enough to cause a small first-degree burn; for localized cold, enough to cause the beginnings of frostbite (i.e. a few living cells lysed by the formation of ice crystals in their cytoplasm). For heat and cold over the whole body, it would have to be enough to overcome the body's natural thermostat, initiating hypothermia or heatstroke.
It occurs to me that I've purposefully endured levels of discomfort I would probably regard as pain with a non-zero value on the torture scale if it was inflicted on me involuntarily, as a result of working out at the gym (which has an expected payoff in health and appearance, of course), and from wearing an IV for two 36-hour periods in a pharmacokinetic study for which I'd volunteered (it paid $500); I would certainly do so again, for the same inducements. Choice makes a big difference in our subjective experience of an unpleasant stimulus.
50 years of torture for one person is probably not as bad as 25 years of torture for a trillion people.
Of course not; by the scale I posited above, 50 years for one person isn't even as bad as 25 years for two people.
If we keep doing this (halving the torture length, multiplying the number of people by a trillion) then are we always going from bad to worse?
No, but the length has to get pretty tiny (probably somewhere between a millisecond and a microsecond) before we reverse the direction.
And do we ever get to the point where each individual person tortured experiences about as much discomfort as our replacement dust speck?
Yes, we do; in fact, we eventually get to a point where each person "tortured" experiences no discomfort at all, because the nervous system is not infinitely fast nor infinitely sensitive. If you're using temperature for your torture, heat transfer happens at a finite speed; no matter how hot or cold the material that touches your skin, there's a possible time of contact short enough that it wouldn't change your skin temperature enough to cause any discomfort at all. Even an electric shock could be brief enough not to register.
The idea that the utility should be continuous is mathematically equivalent to the idea that an infinitesimal change on the discomfort/pain scale should give an infinitesimal change in utility. If you don't use that axiom to derive your utility funciton, you can have sharp jumps at arbitrary pain thresholds. That's perfectly OK - but then you have to choose where the jumps are.
"What's the worst that can happen?" goes the optimistic saying. It's probably a bad question to ask anyone with a creative imagination. Let's consider the problem on an individual level: it's not really the worst that can happen, but would nonetheless be fairly bad, if you were horribly tortured for a number of years. This is one of the worse things that can realistically happen to one person in today's world.
What's the least bad, bad thing that can happen? Well, suppose a dust speck floated into your eye and irritated it just a little, for a fraction of a second, barely enough to make you notice before you blink and wipe away the dust speck.
For our next ingredient, we need a large number. Let's use 3^^^3, written in Knuth's up-arrow notation:
3^^^3 is an exponential tower of 3s which is 7,625,597,484,987 layers tall. You start with 1; raise 3 to the power of 1 to get 3; raise 3 to the power of 3 to get 27; raise 3 to the power of 27 to get 7625597484987; raise 3 to the power of 7625597484987 to get a number much larger than the number of atoms in the universe, but which could still be written down in base 10, on 100 square kilometers of paper; then raise 3 to that power; and continue until you've exponentiated 7625597484987 times. That's 3^^^3. It's the smallest simple inconceivably huge number I know.
Now here's the moral dilemma. If neither event is going to happen to you personally, but you still had to choose one or the other:
Would you prefer that one person be horribly tortured for fifty years without hope or rest, or that 3^^^3 people get dust specks in their eyes?
I think the answer is obvious. How about you?