I also was able to open the hatch on my first visit to Groblar's house wothout the key. The mistake I made was agreeing to kill Sigurd then going to Sigurd's house only to find out he did not kill Groblar's family. At that point he turned on me and I had to kill him. When I returned to Groblar's house I had to then kill him. But both quests are still in my log. I doubt they will go away now. These quests really need a broader branching to allow for more than one method to complete them.
