Sorry for not responding in awhile.
Re: how Spike knew death was a certainty and could have avoided it -- Buffy urges him to leave with her, which I think suggests he could have done so, and he replies that he's "got to" sacrifice himself and "wants to" see how it ends. I think this shows that he's choosing death, not simply succumbing to it.
But could he have actually left? I highly doubt it. He was being burnt from the inside out. You can't just take off the necklace and walk away. He didn't really have a choice in the matter once the effects of the amulet began to show. With that said, I don't think he succumbed to his death. It is certainly admirable that he accepts his fate. I mean, even the imagery of the light from inside his soul being what conquers evil is great, but I think what he did is very much the same as what Andrew and Anya did. They both accepted that they were going to die and fought anyways. And that's what Spike did, he fought, he found out he was going to die, and he accepted that. And for that, he is a hero. But no more than the others there. I don't like that his death was deemed a sacrifice and the most selfless thing ever. Because, in reality, he had no choice. It's just what he did with the situation he was handed that makes it a significant moment.
Re: Spike helping Buffy in season 6, and not simply being attracted to her darkness -- I think we see this from the moment they first have contact in season 6, when he empathizes with her about crawling out of her grave and bandages her hands. One of the first things he tells her is that he'll take care of her, and he keeps offering to do so -- the every night I save you speech, and also, before she confesses to him about having been in heaven, he tells her something about wanting to help her if she's in pain. Of course, he's the one who saves her from basically killing herself in OMWF. And he really wants positive, emotional intimacy with her -- my heart always kinda breaks when, after the first time they have sex, she mocks him with that think-we-were gonna-read-the-papers-and-play-footsie line, since that's clearly what he does want. These are just some examples, but I think this continues until the point where he becomes hopeless about her responding to him when he tries to connect with her in a positive way. But no question, his subsequent behavior in isolating her and playing on her fears is very, very bad; I just think it happens because that's the soulless behavior he naturally reverted to when being loving and kind didn't work.
I agree on the first part. He was definitely helping her pre-OMWF, I just think it's after that where it becomes about him, in his mind. I don't think he sees his affair with Buffy as a way of helping her. It was just "love", or what he thought love was, and he finally got what HE had always wanted. I don't think he saw how destructive it was and he didn't think he was helping her realize her dark side and overcome it or anything like that. He just wanted to be with her, to bring her into the darkness with him
Re: the AR and Spike's punishment in season 7 -- I think a big reason Spike underwent this torture was to get viewers to a place where they could feel like he had paid for the AR. But it doesn't really work, I agree. The AR is fraught with problems. I absolutely understand what you're saying about this. Very, very difficult to put people together in a loving relationship after that happened.
Hmm, I never thought about the torture scenes in that way. But for me, personally, it didn't bring me to a place where I could forgive him. I guess we can agree on that one!
Spell No Spell Demon No Demon
We've learnt from Joss that Humans are just as capable as the bad guys when it comes to this kind of stuff.
True. What I'm saying is that Xander in "The Pack" seemed to be under a greater influence that Spike was under. Xander was under a spell, and yes, it certainly reflects a very true part of who he is, but Spike was under no spell when he attempted to rape Buffy. It was all him, which makes it worse, IMO. Xander's actions in "The Pack" were part of he was, but Spike's AR was all him, not just a part of him.