I have encountered this a few times, and I can't remember the exact cards right now, but if a card says "Look at the top 3 cards of the Encounter Deck. Choose 1 of them and add it to the staging area. Discard the other 2", can you choose to add a Treachery card to the Staging Area?

There is nothing preventing you from adding a treachery - just be sure to resolve it's effects.

In that same vein when a quest set up pulls a treachery card to satisfy "add one card from the encounter deck for each player", the treachery card takes effect?

neillr said:

Yes, you're correct.