This isn't true --- the choice is between N-6 and N-5 people; N-5 people is clearly better. Not to be too blunt, but I think you've badly misunderstood the concept of a utility function.

Yes. The hypothetical utility function could e.g. take a list of items and then return the utility. It need not satisfy f(A,B)=f(A)+f(B) where "," is list concatenation. For example, this would apply to the worth of books, where a library is more worthy than however many copies of some one book. To simply sum values of books considered independently is ridiculous, it's like valuing books by weight. Information content of the brain or what ever else it is that you might value (process?) is a fair bit more like a book than its like the weight of the books.

Actively making your argument objectionable is very different from avoiding the use of the Dark Arts. In fact, arguably it has the same problem that the Dark Arts has, which is that is causes someone to believe something (in this case, the negation of what you want to show) for reasons unrelated to the validity of the supporting argument.

Sorry, I only meant to imply that I had assumed we were discussing rationality, given the low status of the "Dark Arts". Not that there was anything wrong with such discussion; indeed, I'm all for it.