that if the choice were beween 3^^^3 people getting a dustspeck in the eye or one person getting tortured for fifty years, the FAI would always make a choice, and that choice would be for the torture rather than the dustspecks

That is an entirely separate issue. If CEV(everyone) outputted a moral theory that held utility was additive, then the AI implementing it would choose torture over specks. In other words, utilitarians are committed to believing that specks is the wrong choice.

But there is no guarantee that CEV will output a utilitarian theory, even if you believe it will output something. SI (Eliezer, at least) believes CEV will output a utilitarian theory because SI believes utilitarian theories are right. But everyone agrees that "whether CEV will output something" is a different issue than "what CEV will output."

Personally, I suspect CEV(everyone in the United States) would output something deotological - and might even output something that would pick specks. Again, assuming it outputs anything.