The most knowledgeable NPC in the game when it comes to jewelry would be the dwarven jeweler in the Dwarven Halls...
The actual necklace or its location can not be found in the game; the quest is solved by having a copy made.
The Sword of the Gods (potentially the best one handed sword in the game) is in the wastelands, in one of the houses in the abandoned village (in a group of 3 houses on the outskirts of what would be considered the town, IIRC in the southern house).
I'm not sure what you mean by royal court.
The game does tend to get easier as it progresses.