Not many people left that are IDI, I think. There just wasn't time for an intruder to do all that was done that night. But a lot of people go back and forth among the short list of surviving Rs. Including JAR. Maybe Grandpa, too.
Of course, the killer(s) wouldn't have been bungling around in the dark down in the basement...they knew where all the light switches were. I've never read that anyone noticed lights on in the basement. Though the wineceller had no windows, the other rooms did. I've read neighbors mentioning lights left off that ususally were on, and the "weird" moving lights a neighbor saw in the R kitchen. Obviously the flashlight, as the killers moved about the kitchen. I once though maybe it was JBR and BR sneaking down to the kitchen for that infamous pineapple snack (we really don't know whether BR had eaten any pineapple also, do we?) But those 2 kids would not have wiped the flashlight AND BATTERIES down to remove their fingerprints. So that theory is out in my mind, at least. No, that was the Rs moving about AFTER the murder, writing the note, planning their coverup. BR admits to being awake, hearing voices. If his parents had discovered his sister dead in the house, he would have heard more than hushed voices! He'd have heard anquished screaming! And his parents would have rushed in to be sure HE was safe. After all, if this was a legimate intruder murder, how could a parent be sure their OTHER child wasn't lying dead somewhere in the house, even if the note only mentioned their daughter.
And since Burke said he heard "noises", it would stand to reason that Patsy and John would have heard them too and would have checked on them or called the police. But they did not need to since they were the ones making the "noises". And that is why John says I took a melatonin and I wish I had not because I might have been able to save her.