I think Larian should help out those blokes who lose that dagger by putting it in Siva's inventory to purchase and making it reapawn there on a usual trader basis.
But seems like as said above you can proceed anyway. Uninstalling game over such things? lol, it ain't WoW here buddy and these fits won't impress anyone.
There is like a gazillion ways to solve almost every situation in this game, especially earlier chapters. Really it's only Arx where you start getting this "do this or go screw yourself" quests.
