Sorry if you're fed up with this subject already
LEW Guide said:
Information in a characters BACKGROUND is considered PRIVATE unless the player of the character chooses to declare it PUBLIC by stating so.
In my opinion, if a player uses background information and NPCs in a game, that information should be considered public for the current adventure, as long as the DM posts a note to that effect to make sure that the player is aware of it.
In the example, this means that when the player sends necklace away, the DM says, "OOC: By doing this you make Helena public for this adventure. Is that OK with you?" The player then has the option to back out and not send the necklace, or allow the DM to expand the adventure to include what happens with Helena. Or even make a new adventure with this as the main theme (though that'd require the players consent once more).
Even if the DM doesn't want to expand the adventure, the characters background (Helena in the example) might still have changed. I think most players would like it if their NPCs were used as plot hooks, but just like all things borrowed, you should try to leave it back in the same way it was when you borrowed it. When the adventure ends, the player receives "control" of the NPC again, and can evolve it as he wishes - for example, write that her maid called a cleric, who managed to thwart the enchantment.
Ogrins background is public (except for one little piece of information) and if someone would want to create a future adventure where he's hunted by the friend and/or the apprentices of the murdered mesmerizing magician, noone would be happier than me.