Well the conflict here is that you always have to perform as much of the effect as you can. I don't see any reason why this would stop you keeping your conditions with
Deceit if you have
only Shadow conditions in play.
What I'm not so sure about is whether if you have other cards in play you have to choose them instead. I would guess that you don't given you have to declare the response after you've chosen what Shadow cards to discard, but as I said I don't know.
Thranduil