helpo1: Bosper's bow is in the sewers - you need to get the key to them from Thieves Guild assassin. In order to get there, you need to get in touch with the Thieves Guild - you have to either complete all three of their quests (including Nagur's, which you can't), or report at least one of the thieves to Lord Andre.
So, you can still join the Thieves Guild easily, without cheating.
Just report either (or both) Rengaru or Halvor in their respective quests to Lord Andre. Then go to Kardiff. He'll send you behind Halvor's house. Kill Attila and take his key. Go to the barracks-side of the harbor (near Garvell and Brahim), to the crane. Jump to the water and swim to the right. You'll find a door, which you can now unlock.
Proceed through the sewers and talk to Jesper (tell him that you have already killed Attila and that you want to see his leader). He'll take you to Cassia, who will offer you the membership in the guild. However, if you'll refuse (in the contrary to the "peaceful" way - in that case, you can simply walk away), you'll have to kill them, and that can be not easy.
This way has just one "big" drawback - the thieves will want money to teach you things (Ramirez 300 gold instead of 150; Cassia and Jesper 100 instead of nothing).
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PS: I'd recommend you to report Rengaru, because he has nothing to offer you even once you become a thief - just a +1 dexterity bonus. Halvor, instead, will buy silver plates and cups from you with a better price than other merchants.
Very helpful. Now I need to find out why I can't turn in Rengaru or just reload to earlier and do that quest over.
I appreciate everyone's input. This is a great game so far. I can't wait to progress in it. I considered skipping a lot of the city quests and going to Valley of the Mines, but I don't want to give up on so much content and I don't want to gimp myself with missed xp. I'm already looking forward to playing again as a different character type. :)