NPCs pick stuff up off the ground, don't they? So.. try annihilating the whole of Stillwater and loot the corpses. You are boned without the amulet, Raven just won't show up. Though I think someone on TerraArcanum claimed to have beat the game omitting Quintarra completely. I guess you could try forcing other plot related locations by walking right into their exact coordinates on the worldmap.
Hope you didn't also rob the bank in Shrouded Hills. Otherwise, even if you do manage to find the amulet, the herbalist will just shit herself in fear and won't talk to you.
That's some serious oversight and shitty design, frankly. Still an awesome game, but too linear.
Anyway, good luck.