I mostly just lurk, but I've put together a possible narrative that could account for some of the weird discrepancies this case has. All MOO, of course.
Leading up to the murders, we know DS had a court case coming up for break and enter, and we know he had previously had similar charges when he was younger which were dealt with so that he didn't end up with a record. His family knew about that and supported him but I'm sure it was made clear to him that he needed to get himself together and not repeat them.
DS seems to care about his family's opinion of him, if not society's. Even when confessing to murder, he tells police that he had traffic court, which seems to indicate his reluctance to admit he was up for B&E was seriously ingrained in his mind.
So, what I'm thinking is that when he showed up in church, he was thinking of telling his aunt (or a priest, or other family member) about the charges he was facing. That's the bad thing he did. Maybe he was feeling torn between doing the right thing (telling his family) and the wrong thing (acting out his murderous fantasies). He can't bring himself to tell his aunt, so he's goes ahead and kills Hanne.
I do think he premeditated the crimes. The list of names had to have been written before the church times...you don't start writing halfway down a page. They could have been crossed out later. He was thinking about murder before he decided to go to church. It's possible he was motivated by the rumor about money, he could have thought that if he was sentenced to fines and probation, he could pay and not have to fess up to his family.
I think "sleepers for the dogs" is definitely proof he though ahead about killing Hanne. But when the time came, he couldn't get ahold of "sleepers" or just decided it didn't matter. He worked himself up to kill, kicked the door in, and did it. Maybe he looked for the money and didn't find anything, or maybe he was so agitated he forgot. Either way, at that point there was no reason not to go through with his plan for killing Terry, Hailey and Chy.
As for Chy being on the list, if he didn't know she didn't live with Terry I think he planned on killing all of them. It sounds like he thought that being fiends was dating, and he was enraged when she chose the "bad boy" over the "nice guy". (Although he probably didn't ever actually tell her he liked her.) In his imagination she had betrayed him, and "that hideous baby" was living proof. When he crossed off the names later, he figured killing Hailey worked as revenge. He didn't kill Chy and he didn't use sleepers on the dogs, but it was close enough.
Notice that when he first confessed (sort of) it was in front of his dad. Then he kept his mouth shut until Sgt. McCauley got him talking, which he did by basically acting like a family member. That's DS's weakness. Also, I don't think it's surprising that six months later his recollection of Hanne's murder was off.