Yes I think their objective was to make safe the house if you will to the extent that even after they identified the 5 victims matching JB's description of the occupants they continued to cautiously search the house and outbuildings. This makes me think they kept an open mind to some degree about what had happened otherwise why continue searching?
I don't know how much searching was left to do after finding the boys, I think Jeremy's room was one of the only rooms after that to look in. Jeremy could have been wrong about who was in the house and how many it is SOP to clear a structure by looking everywhere particularly since unarmed brass and crime scene personnel would be entering next. In support of your point though the raid team recognized there was a fight in the kitchen and noted that Sheila looked to clean so thus were not fully convinced of the murder suicide theory. They said they didn't push this to DCI Jones though because it was not their place. On TV they show all police working together like a big happy family but in real life things are more compartmental. The best investigators take everyone's expertise into account and ask everyone to assess everything. You will find that many cold cases are solved using evidence that the original investigators had at their disposal but failed to pursue. People get things in their head and close their minds to other ways.
DCI Jones is seen as one such man but in fairness to him he was not in charge when the key evidence in this case was uncovered. After that evidence was uncovered he took part in questioning Jeremy. We don't know that he would have ignored the moderator evidence and Julie's account if he had still been in charge. Some people suggest he would have but they have no evidence to prove he would have. He died before the trial so was prevented from speaking to Dickinson and being able to say to him oops I goofed. That doesn't necessarily mean he refused to face he was wrong.
