The first exception is due to circumstances around magic in the comics. The second is because the ratio of person to demon in vampires was oversimplified by the Watchers understanding IMO and subject to debate but I don't think vampires are incapable of love souless only true altruism and disinterested conscience.I remember that in "Spiral" Gregor, the leader of the Knights of Byzantinum said that if Ben dies than Glory dies. So judging from that information she must be dead. However, as far as i see, information about someone or something are mostly given by certain groups/people and often reflect the point of view of that group/people. Thus Dawn says that she cannot open anything anymore in seasons 6, but in the comics it has been proven wrong. Also Giles and the rest of the scoobies assume that soulless vampires are incapable of love, yet Spikes loves very deeply without his soul at least twice (Buffy, Drusilla). Thus what the knights have said does not necessarily need to be true Additionally, at least for humans, there is an afterlife in the Buffyverse. Likewise there might be an afterlife for other species, like demons or gods. So there are chances that Glory is actually alive in one way or an other. And i wonder about the other questions that have been posted as well.
If the Master could be brought back from the dead by his associates by sacrificing the humans near him in "When She Was Bad", they could have written something similar for Glory, complete with the assistance of Dreg and Jinx to bring her back. The fact that they didn't was good judgement on the part of the writers. I was never all that keen on "The Gift" because of the tenuous connection between Buffy sacrificing herself due to Dawn being made from Buffy's blood. Sorry, but if Dawn was made from Buffy to the extent that the sacrifice of one could stand in for the sacrifice of the other, wouldn't she look more like Buffy?I have a theory about Glory: when Ben dies, Glory is only weakened (or I thing is the world for "affaiblie"). She can't materialize anymore, but I think if some magic could bring her back to life it could be. It's a theory of course, but I thought to myself, it would have been interesting to bring her back (maybe in another form).