Killing the dragon is fine once you get the Mavin Sword. : )
I am clueless as to your dagger problem. The guild usually gives you a quest to kill Grue Morde in order to become an assassin so check your journal and see if you have an assassin in waiting in your party! If you just want the dagger back you would have to reload or have someone hack one into your inventory.
Going back to The Serpent Temple is the only way to get Serpin Coins. The Pawn Shop is the only place where you could have sold them and gotten them all back. Making more without entering the Serpent Temple would require inventory hacking again and I am ignorant of the details as to how to go about it in either case.
Moni
|