Regarding the line I've bolded, I wish I could agree with you, but I really do feel it is just sloppy work by the crown. It would have served a very useful purpose, showing how IS changed his story.
We know Oliver leaves for work at 7:15am. I think it very unlikely that he works 8am to 5pm, and more likely that he finishes work at around 4pm, with an early start. Even if he dropped in to his girlfriend's house to pick up his satnav on the way home, he could have been home by 5pm. Which is when IS says he arrived home and found the note. Who got there first? If he was first, was the alarm not set and is that normal? Boris didn't greet him, wasn't that unusual? Where did he think Helen and Boris were?
Just a few simple questions, to get to the truth, which is what the court needs to know.