Okay, this has bothered me for a while now: how did rose and dave get to the trolls? When one ascends shouldn't they revive on skaia? And what did the green sun have to do with that???

The explosion that created the Green Sun is what killed them.  They died on their Sacrificial Slabs so they revive where they are. The trolls followed the light of the Green Sun to meet up with them out in the Furthest Ring