This probably can't be answered, but is there any chance the killer tried to get into other houses, maybe even An's house, and the Miyazawa's were just unlucky that he managed to get into theirs?
I've also been looking at the pictures of the house. Am I misinterpreting the images, or is the driveway/garage kind of in a pit that sits lower than the rest of the house? I ask because to me it looks like the balcony is higher, and possibly harder to reach, than it appears to be at first glance.
If the killer had climbed onto the car you would think there would be some evidence of that, but I'm not sure the TMPD has ever said anything about it? There are some objects near the garage which could have been used as a step, but it's hard to tell how solid they would be.
I've made this quick scribble to show what I mean about the garage "pit" being lower compared to the front door:
View attachment 488488