Oren's location and those of the plants you need are all shown on
this map of the northern crystal forest. Oren shouldn't wander too far from where you first met him. The flowers will only be there after you talk to Raven, so you may have to run around areas you have already cleared, possibly holding down the alt key.
Explore the area a little ways south of the Raanaar library. You will find a pool of lava and the solution to this quest, at number 7 on
this map.
The DK was either playing along to get close to the witch (more likely), or was much better able to resist a command that would result in his own death than a simple 'come here' command.
If you told the ghost in the crypt that you were seeking the amulet for his daughter, he would have warned you that the amulet was cursed and you would then have the option of telling the merchant that. The merchant's curse has no effect, so some people prefer not to warn him, to get the loot he drops.
