Yeah, the scorpion traps are almost like cheating
I agree, put off going to the castle as long as you can. It's going to turn nasty and you need a fairly strong character to make it through. My first time in the game I went much too early and finally had to restore an earlier save.
Blake won't give you the key until you build up your rep and do something spectacular, such as the orc quests from the captain or saving the boy from the vampire.
The Verdistis sewers are mainly optional; they're meant for gaining experience and treasure. They have nothing to do with the plot itself.
Nightowl2