Some of my theories on survival:

Burt: Everyone on the roof agreed that before Burt's zipline broke, he was very close to the other building. As a long-time climber who's used ziplines before, I can tell you that forward momentum would have carried him into the side of the building he was zipping into. He wouldn't have been on his back as some mentioned, nor on his belly (Superman style), but rather in a sitting position. Angel himself says that he only pulled up about 10 yards of rope. If Burt was close to the other building when he fell, he would have had the brunt of the rope on his side and the burned/frayed end of the rope would have been well behind him. It's very possible he could have grabbed the zip line, bounced into the side of the building, and fallen a much lesser height than we assume.

Angel: His predicament sounds more problematic, but no less survivable. The brunt of the damage to the building was to the lower floors. Those would have been the ones to collapse fully. I think we're wrongly assuming a building collapse like we saw on 9/11, but only majority of the center of the roof collapsed, probably because of fire traveling up the elevator shaft which is always in the center shaft of a building. It doesn't sound like the upper floors were too much impacted by the fire below, because remember Angel was able to descend into Bill's apartment and we weren't hearing fire noises. In addition, Hope was nearby and he didn't have to fight fire to get to her. With that in mind, the lower floors of the building could have collapsed, but that doesn't mean everything collapsed in a pile of dust, and Angel could have easily survived in a pocket of undamaged building. There have been plenty of documented cases of this in real life situations, and I'm sure our brilliant show writer knows this.

Saul and Victor: Last we heard from them, they sounded like they were fine (relatively). If they're smart, they'll lay low and bide their time, follow the Mallers and see what they can do tactically without giving themselves away, while at the same time trying to re-establish contact with Michael and the rest of the helicopter survivors.