Buffy wears the amulet and Spike leaves her to go up in flames alone. Somehow I don't think that would have happened. In my Spuffy heart Spike would have stayed with Buffy and gone up in flames with her.
More likely, Spike would have slugged Buffy, tossed the amulet into the pit with what was left of the ubervamps, and ran out of the hellmouth with her draped across his shoulder as the cavern collapsed, sealing it forever. On the way out, he would have torn down some heavy drapes for protection (remember, it's broad daylight outside) and got out just in time to see the buss roaring away with the Scoobies and the few surviving Potentials.
Thinking quickly, Spike would have run to a car parked and abandoned conveniently close by with the keys still in the ignition and equipped with heavily shaded windows (Hey, this is MY version of what happened. If you don't like it, make up your own) tossed Buffy in and sped away, taking a different route out of town that that taken by Wood driving the bus. Spike's newly acquired car is a supercharged sports car, so he has no trouble outrunning the collapsing hellmouth. Once safely away, he pulls over in a well shaded area, just as Buffy is regaining consciousness.
"You hit me!" Buffy says, rubbing her bruised and clearly swollen jaw.
Spike nods, "Yeah, well, I was kinda afraid that dyin' the third time might make it permanent, luv. Can't have you doing that. You're the one, remember?" He climbs out of the car, looking back at the cloud of dust just beginning to settle over what was only moments before the closest thing to a home that he had known in over a century. Buffy comes and stands beside him
"No," she says, moving closer, putting her arms around Spike's neck and pulling him down for a significantly long kiss. "You're the one. I think that I've known it ever since that night when I killed that first ubervamp and took you back home. I was just too busy trying to save the world to notice."
"Buffy," Spike begins to protest, bur the Slayer cuts him off with another kiss. This one lasts longer than the first one did.
"There's a whole army of Slayers ready to protect the world now," Buffy says, "I can retire and settle down somewhere quiet. Have a life with someone I love." She kisses him again. "Someone with bleached hair and a long black leather coat.
"They must think that we never made it out before the Hellmouth collapsed." Buffy said. "Let's not tell them any different. Not right away, anyhow. Let's just go someplace where we can be alone, no monsters, no Watchers, and no Pot... new Slayers, "she corrected herself, "to have to babysit. Someplace where we can..." she whispered in Spike's ear, grinning wickedly. "I'll bet we can even find some handcuffs somewhere, just to make things interesting."
Grinning, Spike says, "I know a place that would be perfect. A bit of a drive from here, but well worth the trip. Ever been to New Mexico, pet?" as they are climbing back into the car."
"New Mexico?" Buffy asks. "No. What's in...?"
"Little place called Carlsbad Caverns, luv. The unexplored parts. Unexplored by humans, at least. Miles and miles of tunnels and chambers, underground waterfalls and lakes, phosphorescent minerals in the cave walls so that it's almost as bright as day in some spots..." The car roars off down the road. headed east.
