Are you saying the negative utility incurred by a dust speck is zero?

Nearly zero. That's part of the hypothesis: that it be the smallest possible unit of suffering. If the logarithmic scale of quantification for forms of suffering holds true, then forms of suffering at the maximal end of the scale would be practically infinite comparably.

Either a quantity is infinite or finite.

Correct, but a number that approaches infinity is not itself necessarily infinite; merely very large. 3^^^3 for example.

You've completely lost me. If X is the negative utility of N dust specks, and Y the negative utility of fifty years of torture, then the first sentence implies that X > Y.

The negative utility yet considered. Also, keep in mind that we're at this point priveleging the hypothesis of torture being chosen: we are allowing the number of speckings to be adjusted but leaving the torture fixed. (While it doesn't really change anything in the discussion, it bears noting for considerations of the final conclusion.)

Then the second sentence defines a second kind of negative utility, Z, due to other consequences. It goes on to imply that X + Z < Y.

No, it implies that Z(X) + X < Z(Y) + Y.

so there's a contradiction somewhere, unless I've misread something.

My argument rests on the notion that the Z-function value of X is effectively zero, and my further assertion that the Z-function value of Y is such that it overwhelms, when added to Y, the value of X.