Be that as it might (and I do agree that whether Spike wanted to save the world or not, him leaving Buffy to fight alone doesn't make much sense), a good writer doesn't force their characters to do something just because the genre conventions demand it. There needs to be a reason for a character's actions. This is a trap in which Joss fell again and again, especially with comic relief characters like Anya, Harmony and Andrew.
You mean I dreamt up Joyce dying, Hank abandoning Buffy, Xander's troubles in finding a job, Buffy, Willow and Xander being the school outcasts and the myriad of other very much real life problems that the characters had to face in previous seasons? Just because they failed utterly to deal with their problems in S6 doesn't mean they have never faced such issues before. Heck, the whole seasonal arc for Buffy is pretty much "When She Was Bad", only excruciatingly slow and about as interesting to watch as drying paint.
Technicality? Really? That's an odd thing to say considering the entire show is about this "technicality". Vampires are killed for being vampires, I wouldn't call it a technicality, it's certainly big enough factor to justify killing them and sparing human characters who have done the same. Even more importantly, no one in the show bothers to explore the ramifications of Spike being so "human". Even after he manages to get his soul back they continue to kill vampires. It's an exception for the sake of exception, not a gamechanger.
As for the symbolism - things need to make sense in the story itself too. For instance Angel is obviously a metaphor for bad boyfriend in S2. Does that mean the writers should have refrained from having him kill anyone since the run of the mill bad boyfriend isn't a serial murderer?
No, I meant the internal consistency of the story. If in one episodes vampires need an invitation to come in, then in another they don't without any explanation offered, that's an inconsistency. Same with vampires first being all about evil, then for no rhyme or reason, some become capable of "love" and getting back their soul. I have nothing against trying something new but a decent writer does that without violating the internal logic of their story. Any idiot can write a story that's contradictory.
Nice try but that's not really what happened. Anya without her powers is pretty much like Spike with the chip. Her mentality didn't change, she still wanted to smite people and missed her murderous ways. She was only changed by her love for Xander. And she later chooses to stop being a demon. I am not saying it was a well told story or even one that I like but it was basically the same story as Spuffy, only with 95% less angst and nakedness and dumb speeches. And it lacked the great stumbling block of Spuffy since Anya was human, so whatever changes occurred in her they didn't violate the mythos.