The Mezzanine

I think I don't understand (in the context of the show, not just in your fic) why Buffy coming back the second time is so much more earth-shaking than coming back the first time.

For that, I refer you to stormwreath's meta. Particularly this bit:

Eventually, the forces protecting the binding spell on the Slayer Spirit became disrupted by the event long ago foreseen by the Guardians: the mystical resurrection and calling back from Heaven of a dead Slayer. As long as she was [still] alive, the binding spell was overloaded and so vulnerable to being broken. However, this posed a huge risk to humanity. If there were no living Potentials at the moment the binding was broken and the actual Slayer was then killed, the Slayer Spirit would be completely released. No more Slayers would be called, ever.

(What would have happened in the hypothetical situation that there were no Potentials alive at the death of a Slayer before Buffy's resurrection? Presumably, the Slayer Spirit would have waited around, caught by the binding spell, until a potential-Potential hit puberty, then immediately empowered her as the Slayer. I'm assuming that it took The First a vast expenditure of Its power to even attempt to kill all the Potentials at once, especially since new ones are constantly being born. This Whack-a-Mole game was worthwhile for It in season 7 because if It ever once managed to get rid of all of the Potentials for even a single moment - then kill Faith and Buffy - there'd be no more Slayer ever. Before the binding spell was broken, it would have had to keep on killing the new-born Potentials forever; and It lacked the strength to maintain Itself and the army of Bringers It would need for such a huge task on a permanent basis.)

The general idea being that Willow hasn't just resurrected her friend; she's resurrected the Slayer. While there already is a Slayer...