Anya spends the next episode she is in after Hell's Bells trying to get Xander's friends to wish him dead so she can kill him. She still hasn't come to terms with her past as a vengeance demon -- and she needs to be apart from Xander in order to do that,

A very good point. I think what Anya really wants is support, moreso than vengeance (she stops Spike from making a wish to hurt Xander in Entropy) Particularly from the other women; Tara is the only one who gives her a hug when she sees her again for the first time after the wedding (just as Anya was the one to embrace a distraught Anya in Bargaining.) Being left at the altar, IMO, is a bit like what happens in most instances of RL divorce: usually (statistically speaking) the woman's income goes down while the man's goes up.

Being left at the altar has huge consequences for Anya - the shared income is no longer there, the home, the friends and family all taken away (Willow and Buffy's first loyalty is to Xander, and of course it is; he received very little flack from them when he comes back to town. It's Spike who gives Xander the real dressing down in Normal Again; part of that is Spike's natural, if often twisted, sense of chivalry; a little bit of it may be jealously that Xander rejected an option utterly unavailable to Spike as a demon.

Sorry to go on about Anya like that rather than Xander, I'll definitely think on your observations further - I suppose we've just done a bit of a joint meta in that regard?