I don't think JD left the garage unlocked. She's fearful, and has already talked to family/friends about what to do if she doesn't show up for appointments, pick ups, meetings, etc. She is desperately trying to think about every example of how FD is going to hurt, kill, kidnap kids, sabotage her family. She Always knew what she was dealing with, it was the LE that underestimated the danger to JD.Good theory, but how would have FD have gotten into the house? Maybe she left the garage door open when she went to school, but would she have left a door unlocked? I really don't think so.
Now, back in the day when I was dropping kids off at school. Sometimes I forgot my purse. It happened more than once. However, JD had a busy day planned. Was she showered and ready to go, stopping home for a couple things? Or did she usually drop kids off and come back to shower, and get ready? Least amount of steps is to ambush JD in or near garage IMO. However, we are dealing with the 3 stooges leaving a bloody trail of evidence all over CT.