Okay, gonna back up fly on the wall more substantially now.
Other types of demons, while indeed a much more complex ethical issue, are irrelevant. The term "demon" is really used too broadly. Some are clearly good or benign and must therefore have some type of soul, though obviously it wouldn't be a human one. Others range from crafty, sadistic fiends to barely intelligent monsters. Sometimes the difference isn't readily apparent, as Angel was unfortunate enough to discover the hard way in "Judgement".
But soulless vampires are the issue here, and there is no such gray area with this type of demon. There can't be, not even for one of them. Otherwise, as we've said over and over, Buffy wouldn't be able to get away with her "slay first, ask questions never" policy. There can't be a possibility for a soulless demon to redeem itself. They are always irredeemably evil at their core, because their core is this type of demon. They lose all inherent goodness the moment their souls leave their bodies. The only "humanity" remaining is memory, neutral abilities and skills, and selfish human vice.
Regardless of his exceptional circumstances, Spike does not get to be a special exception, because you cannot assume that he would be the only one who doesn't fit the pattern (or wouldn't fit, given the chance). If he doesn't fit the pattern, then the pattern is a lie, and Buffy shouldn't be slaying vampires on the assumption that they're irredeemably evil. Fortunately, they are. Just because one of them is occasionally helpful, disturbingly obsessed with her, and unwillingly prevented from hurting humans, it does not make him less irredeemably evil. As Giles said, "A vampire isn't a person at all. It may have the movements, the memories, even the personality of the person it took over, but it's still a demon at the core. There is no halfway."
The chip isn't a substitute for a soul. It doesn't even come close. Just because some creatures with souls choose to commit crimes, it doesn't mean they couldn't have chosen differently. That is why we are able to subject them to justice, because they had the ability to choose better, but didn't. Soulless vampires don't have that ability.
Spike's chip is a shock collar. All it does is indiscriminately punish him for harming humans, not even making exceptions for when it wouldn't be evil to do so, like for self-defense or to protect someone else. He is allowed no choice in the matter, and clearly hates it. Any love he claims to feel is the imaginary creation of his romanticizing tendencies. He proves time and time again, both with Drusilla and later with Buffy, that the "love" he feels is all about him. He tries to change them or forcibly persuade them if they are unwilling to reciprocate. Getting his soul is just one more way of trying to *make* Buffy love him. It isn't to redeem himself, it's to escape blame. He wants a free pass for everything he did to hurt her, and he wants to nullify what he considers to be the only factor in her inability to love him (his soullessness). It is a reckless, foolish, and selfish action, no matter how positive the outcome proves to be, and it's even a little cruel, because it denies her the option of hating him for what he did to her (or tried to do).
He succeeds, and that's exactly what happens. Once he has a soul, he *should* get a free pass, because the soul had nothing to do with any of that, and the soul is what really matters. Nothing he did before he got it counts on his balance sheet now, but he and Buffy still have to deal with the fact that it all happened. The only problem is that the Spike Buffy needs to deal with so that she can properly get past it all doesn't exist anymore, and she's left feeling obligated to take responsibility for the Spike who exists now.
Until he gets his soul, his best bet for survival is relying on the Scoobies' ill-founded (and completely contrived) tolerance of him. By venting his urge to kill on demons, he makes himself an enemy to them. Only the protection of the Scoobies and the Slayer keeps him from getting in serious trouble with the demonic community. He has no choice but to stay in the Scoobies' good books, even when Buffy is gone. Selfish motives can be found at the root of everything he does, even if the effect benefits someone besides him. Everything he does to "prove himself" to Buffy is not done selflessly, but out of the erroneous expectation that he should and will receive a reward for it--which would include anything from not getting staked, to being allowed to be (sort of) included in the group, to Buffy admitting having feelings that he's so convinced are really there.
Fortunately, once he has a soul, this is no longer a problem, because that's what having a soul means. His selfish motivations can finally be replaced with pure ones. Not that they all are, because the ability of soulful beings to be selfless does not diminish their ability to be selfish.
It's much easier to sympathize with something soulless when it wears a human face, which is just as true of the audience as of the other characters. The Scoobies don't kill him while he has a chip because the writers wanted to keep him around because they are (grudgingly) extending mercy, something that only their souls allow them to extend. It isn't a logical decision. The logical (and also safer, smarter, and more ethical) decision would be for them to realize that he's much easier to get rid of if he can't fight back, so stake him before he can find a way to get the chip out. They couldn't bring themselves to kill vamp Willow either, and she wasn't even remotely harmless. They couldn't have known she would be staked seconds after returning to her world; for all they knew, she could have ended up killing thousands of people there, but still they let her go. Like her, Spike was able to prey upon their sympathies by seeming helpless and pitiable--and, eventually, helpful. He even did it in "Becoming" and "Lovers Walk"; Buffy let him go, even though she must have known that he would go right back to killing people once he left town. And then she lets him go again in "The Harsh Light of Day". Keeping him alive never has anything to do with believing he is capable of changing.
And I don't think your theory about vampires works, frankie. The turok-han were never human to begin with. They could easily have the exact amount of demon in them as human-based vampires (which is enough demon that Angel's was able to destroy a powerful demon like Eyghon via inner battle within seconds), but they don't have any of the memories, neutral skills and abilities, or selfish human vices that flavor what human-based vampires become. All they have is bloodlust and a desire to kill, plus the strength and senses to make them really good at it.
We don't know what kind of demon's essence the Shadow Men used to create the first Slayer. It wasn't necessarily a vampire demon. Even if it was, the effect of inhabiting a living, breathing, soulful human being on such a demon would be very profound. Bloodlust would be rendered unnecessary, because humans make their own blood. Lust for the kill would be directed against other demons, already the natural enemies of the human. It wasn't necessarily tamed naturally over time. The Shadow Men's descendants (or perhaps the Guardians, who seemed to care a whole lot more about the well-being of the Slayer herself) may have used magic to fine-tune it until they had the human/demon combination that worked best. (Trial and error: as crucial to dodgy magic as it is to science.) And in any case, it isn't until Buffy gets resurrected that Spike can physically hurt her while his chip still works, so whatever demonic quality is the source of her power (if it's still even demonic at all) isn't significant enough to register with it.