Abraham was very very high profile in his area. So everyone around him would have watched this whole story take place.

It wasn't a later minute change of hearth, it was basically a dramatic way to drive home the message DO NOT SACRIFICE humans!

It's got the "hook" "Abraham is going to sacrifice", to get everyone's attention. And then the powerful message "don't do that".

I've heard different interpretations, and we have multiple versions of the story. In the Quran, the boy is Abraham's other son, Ishmael.

I've always like the interpretations of Genesis (and other old ME epics) as folk memory. Myth as a way of telling history allegorically.

For example, people named in genesis often represent tribes, ethnicities and nationalities. Their names were literal personifications of contemporary peoples. The binding of Isaac represents a transition from human sacrifice to animal sacrifice, as a symbolic substitute continuing the same tradition.

I've even heard interpretations of the fall from eden where eden represents a hunter gatherer lifestyle in the fertile crescent, and the fall represents settled, agricultural life.