I think you mostly got it right.
I took the prologue to be an actual dream (or daydream) and I still think of it that way, but maybe a different interpretation will become possible later in the book 
This is the only part that you got wrong, I think. She does say something about hearing the truck during holidays when she was in primary school, but the scene itself takes place after she has already attended and has stopped attending middle school (こころは学校を休むようになって). The reasons for that will become clear later.
If you haven’t yet, check out the first week of the book club for some elaborate discussions of that first part.