That person certainly has fundamental value. I suppose you could argue over whether it's the same value as a "normal" person
You don't really have to
suppose that we could argue whether a lump of human-shaped meat that will never, ever have brain function doesn't have the same fundamental value as a living, breathing person. We assume that every time we unplug one. It's fine for you to disagree with this, but your assertion was that everybody sees fundamental, unconditional value in human life, and this is demonstrably untrue. The abortion debate wouldn't even exist otherwise.
But if, for instance, a pedophile were to use that infant to pleasure himself, you would certainly agree that is disgusting and wrong
I do think that's disgusting and wrong, because pedophilia is morally revolting. I don't think it's doing the braindead infant any harm, though. It's just fucking gross, and I don't like it, and I believe that normalisation or enabling of pedophilic sexual urges is going to lead to the abuse and victimisation of other, living children.
The term "soul" is just shorthand for the idea that human life has inherent value
It absolutely is not that. The soul is the immaterial essence of a human being, one that can retain our aspect beyond our material form and continue existing with some level of perceptual identity in an afterlife.
An alien race kidnaps you and puts you into a chamber with two buttons. The aliens have a giant death laser pointed at the Earth from an extreme distance--no one on Earth has any idea that they have it, or that the aliens even exist. The aliens tell you that you have one minute to press one of the two buttons, after which they will execute you in a painless fashion. If you don't press one of them, they will torture you relentlessly in unimaginable ways. One button will fire the death laser, instantly destroying the Earth and all of humanity with zero awareness or pain for any of them--just boom, gone. The other button disables the death laser. The aliens don't care which one you press, won't interfere with Earth further one way or the other, and are doing this for the lulz. Which button do you push?
I'd press the one that doesn't destroy earth, because I have a pretty strong sentimental attachment to the place. I understand your point is that when I'm gone, all of that stops mattering and by my own logic it should make no difference to me whether I destroyed humanity or not on the way out, and I do maintain this view, but all the same I'm still personally invested in the life I've got going on and am going to lean towards trying to protect the things that I value and enjoy for as long as I can, even if I know my efforts are ultimately futile.
But what I'd rather ask here is why pain and suffering matter to you if human life itself doesn't matter. Why do you care if a human suffers, if you don't even care whether they live or not?
Because pain hurts, and I have empathy. I'm okay with not existing, because it's how I spent most of the last 13 billion years and I don't recall it being a particularly bad time for me. I can't empathise with the dead, but I really wouldn't like it very much if somebody shot my mother, or raped my wife, or nailed my testicle to a wall, and I would empathise with anybody else that happened to.
I think the central point you're trying to make is that if I don't agree with you that human life has fundamental value, I must not be assigning any value to anything whatsoever. If so, I think this is a pretty self-centered take that will make it difficult for you to understand disagreements. If not, you might need to explain your point better, because I'm not getting it.