So many people seem to be trying to find ways to dance around the simple plain issue of whether we should consider the multiplication of small disutilities to possibly be morally equivalent (or worse) to a single humongous disutility.

On my part I say simply: YES. Torturing a person for 50 years is morally better than inflicting the momentary annoyance of a single dust speck to each of 3^^^3 people. I don't see much sense in any arguments more complicated than a multiplication.

I agree that this is the critical point, but you present this disagreement as if multiplying was the default approach, and the burden of proof fell entirely on any different evaluation method.

Myself, however, I've never heard a meaningful, persuasive argument in favour of naive utilitarian multiplication in the first place. I do believe that there is some humongous x_John above which it will be John's rational preference to take a 1/x_John chance of torture rather than suffer a dust spek. But I do not believe that a dust speck in Alice's eye is abstractly commensurable to a dust speck in Bob's eye, or Alice's torture to Bob's torture, and a fortiori I also do not believe that 3^^^3 dust specks are commensurable to one random torture.

If John has to make a choice between the two (assuming he isn't one of the affected people), he needs to consider the two possible worlds as a whole and decide which one he likes better, and he might have all sorts of reasons for favouring the dust speck world - for example, he might place some value on fairness.