An impressive analysis of this riddle. However, you overlooked one important factor: the amount of detail that the boy was able to see through the window. Could he really see "rain falling on the lawn" from the western half of that aisle? If the library were located on the second or third story, he would most likely need to be much closer to the window to view through it at an angle that would display the ground level.
[Also unsigned, but disguised]