Fair warning! Chapter is an author's note. Yes, I know, some of you hate my author's notes and don't understand why I include them. I do it because they explore topics and issues relevant to the story and the show and because I and some of the readers/reviewers want to discuss those issues. You are as always, free to skip them or join the conversation. But remember to keep your comments polite.
A/N Syg, your theory is interesting, if unprovable, given we don't get an explanation for Giles' attitude from the show, but it would explain Giles' attitude toward Xander, his casual dismissal of him and him basically ignoring Xander. I always thought Xander maybe reminded Giles of himself. Angry teenager lashing out occasionally and both had a 'Ripper' aspect. Giles' past made him extremely uncomfortable and like Xander wanting to forget everything that happened while he was possessed, Giles' past was something he very much wanted to forget or at the very least ignore as much as possible. Having a teenager that reminded him of himself, who simultaneously embraced and succeeded where Giles failed, could be a breeding ground for resentment. It's very human, but I personally, would have preferred Giles mentor Xander, to make sure he didn't repeat Giles' mistakes. Of course, he should have also mentored Willow, because, like Giles himself, she ended up misusing Magic and causing a lot of pain and misery along the way.
By the way, I personally liked that Ripper aspect, especially in Xander and I would have loved to have seen more of it. When he stared down Angel, Angelus, Jack and I have found comments from Joss that he should had Xander doing that a lot more because the actor did it really well.
But, yeah, an unacknowledged resentment of Xander succeeding at something big brained Giles thought was impossible could happen. A reviewer on an earlier story of mine commented that Giles initially going along with Buffy's Cruciamentum could have been unresolved anger at Buffy over Jenny's death and Buffy's interference in Giles' revenge, as well as hiding Angel's return, both of which are things Giles yelled at Buffy about. Giles going along with the Cruciamentum in the first place never made sense and some kind of explanation is needed to try to understand it and since we don't get any from the show, we have to try to find an answer ourselves.
And I totally agree, it was bad writing to turn Anya into 'wivey' and give her very little else to do. But they were trying to shoehorn her into a role that was supposed to be another character's, Cordelia's and mostly ignored the vengeance demon and centuries of living she'd had before in order to do it. And here's where there's another disconnect between the writers and the audience, the writers ignored her past and the audience was fixated on it and as a result the audience thinks Xander's being hypocritical in also ignoring it. But the audience and Buffy want to ignore Angelus and Spike's pasts as well, just forget about them. And yes, if the writers had acknowledged Anya's history, she could have replaced both Giles and Willow on the show. Her demon and supernatural knowledge had to far outstrip Giles' and she was a witch before she was a vengeance demon. An extremely powerful one at that. She was Norse, as in Viking and she'd turned Olaf into a troll. In Viking tradition, trolls are demigods, the kind of power to do that, would definitely outstrip what Willow was capable of at that time.
Until the writers decided to derail their happy ever after and Xander gets mentally and emotionally tortured on his wedding day, leaves Anya at the alter and she goes back to being a vengeance demon, like they suddenly remembered, 'oh! she used to be a man killing demon! (Marti Noxen said in an interview 'We punish happiness.') Buffy and Angel can't ever be happy together, the curse won't let them and the soul is something Joss freely admitted was nothing more then a plot point, a way to appease the audience and get Buffy together with those characters, but that exact plot point also made it impossible for Buffy and Angel to ever be happy either together or apart. If they're together, curse breaks, Angelus tortures and kills everyone. If they're apart, they can't be happy with anyone else because each loves the other so much. And Angel is a shadow over a relationship with Spike, a constant reminder that she loved him first and deepest. That kind of knowledge would eat at Spike, wouldn't it? It certainly ate at him when Drusilla would cheat on him with Angelus. She had some kind of feelings for him and there's Spike, the only two women he's ever loved and both of them loved Angel/Angelus first. I don't see how it couldn't eat at him, it certainly ate at Riley.