Question about your hypothetical: What happens if less than the necessary number of people volunteer for the dust speck? Do they A) get the dust speck to no purpose, or B) is their speck count reduced as though they hadn't volunteered?

I wouldn't volunteer either way, if you ignore irrational guilt trips like ArisKatsaris said. If there are 3^^^3 volunteers, they are irrational in situation B, and probably in situation A as well.

Would you rather that 3^^^^3 people got one less dust speck in their eyes (in times when dust specks were not the limiting factor on much more important activities), or prevent one person from being horribly tortured for 50 years?

That's the same question effectively, so the former.

A related but similar question would go: Would you volunteer not to have your dust speck count reduced, with it understood that A) if 3^^^3 people volunteer for this, someone will not be tortured, and B) there are well over 3^^^3 people being asked this, so it's not well beyond futile.

Yes, I would volunteer for this, but that's just because I can rationally anticipate that if I denied so volunteering I'd be irrationally having guilt-trips over this, which would be significantly higher disutility in the long term than a dust-speck. In short I'd be comparing a dust speck to the disutility of irrational guilt, not the disutility of torture/3^^^3