Recovering irrationalist: in your induction argument, my first stab would be to deny the last premise (transitivity of moral judgments). I'm not sure why moral judgments have to be transitive.

I acknowledged it won't hold for every moral. There are some pretty barking ones out there. I say it holds for choosing the option that creates less suffering. For finite values, transitivity should work fine.

Next, I'd deny the second-to-last premise (for one thing, I don't know what it means to be horribly tortured for the shortest period possible -- part of the tortureness of torture is that it lasts a while).

Fine, I still have plenty of googolplex-divisions left. Cut the series as fine as you like. Have billions of intervening levels of discomfort from spec->itch->ouch->"fifty years of reading the comments to this post." The point is if you slowly morph from TORTURE to SPEC in very small steps, every step gets worse because the population multiplies enormously while the pain differs by a incredibly tiny amount.