A bounded utility function does help matters, but then everything depends on how exactly it's bounded, and why one has chosen those particular parameters.

Yes, and that is my precise point. Even if we assume a bounded utility function for human preferences, I think it's reasonable assume that it's a pretty huge function. Which means that antinatalism/negative preference utilitarianism would be willing to inflict massive suffering on existing people to prevent the birth of one person who would have a better life than anyone on Earth has ever had up to this point, but still die with a lot of unfulfilled desires. I find this massively counter-intuitive and want to know how the antinatalist community addresses this.

I take it you mean to say that they don't spend all of their waking hours convincing other people not to have children, since it doesn't take that much effort to avoid having children yourself.

If the disutility they assign to having children is big enough they should still spend every waking hour doing something about it. What if some maniac kidnaps them and forces them to have a child? The odds of that happening are incredibly small, but they certainly aren't zero. If they really assign such a giant negative to having a child they should try to guard even against tiny possibilities like that.

Also, are they all transhumanists? For the typical person (or possibly even typical philosopher), infinite lifespans being a plausible possibility might not even occur as something that needs to be taken into account

Yes, but from a preference utilitarian standpoint it doesn't need to actually be possible to live forever. It just has to be something that you want.

Does any utilitarian system have a good answer to questions like these? If you ask a total utilitarian something like "how much morning rush-hour frustration would you be willing to inflict to people in order to prevent an hour of intense torture, and how exactly did you go about calculating the answer to that question", you're probably not going to get a very satisfying answer, either.

Well, of course I'm not expecting an exact answer. But a ballpark would be nice. Something like "no more than x, no less than y." I think, for instance, that a total utilitarian could at least say something like "no less than a thousand rush hour frustrations, no more than a million."