Ultimately fandom prioritizes him in any situation - and the show oddly enough follows suit, by making his redemption more important, bigger and shinier than Buffy's; by making him the one who saves the day thanks to his sacrifice and the amulet from Angel.

I wonder how much of that was the writers giving fandom what it wanted, and how much of it was based on the writer's feelings independent from the fandom.

In the shooting script it's implied that the Slayer Spell is what actually unlocks the amulet's power and Spike is then connected to the new Slayers - he feels their pain as they are injured - but that doesn't come across in the episode itself.

I didn't know that. That's a cool detail that I wish they'd kept in.