I don't think they knew that S7 was the last season... not until about halfway through filming. SMG announced that she was done in like Feb after that season started, if I recall correctly. (And didn't inform anyone on the cast - they found out via the Entertainment Weekly interview.)
There are characters far more lackluster than Dawn. And without Dawn, there wouldn't have been much of a S5.
I'm not a big Xander fan, but I prefer him to Anya - she was an annoying character (though, less so than Riley). Xander is the guy who sees things. He's there, on the outskirts, the one on the outside looking in. He's the one no one pays attention to because they don't see him as a threat. He has some annoying qualities, but he'd a hugely boring character if he didn't.
Leaving Anya at the altar, he did because he thought it was the right thing to do. Sure, it hurt, it was painful, but he thought he was doing it for the right reasons. Even if the 'future' he saw wasn't real, he was having big doubts about the entire thing and would rather not do it, than make Anya miserable by going through with it. From his POV, he was choosing the lesser of two evils.
Spike was a villain in S2 when he first showed up. He wasn't supposed to be a recurring character, but he was so hugely popular that he was worked into the show. Later, he wasn't so much a villain as a foil for the white hats. He's the ambiguous presence - he's not really good, but he's not really evil either - no matter how much he tries to jump on one side or the other.
I'll have to jump back into this later... got some other stuff to take a look at....