How could Abraham ever have believed that G-d wanted him to sacrifice his son?
I think that the answer is in a drush : Vayaar et hamakom me-rachok - "And Abraham saw the place from a distance..'
Read 'Hamakom' in its sense of being a synonym ('kinnui') for the name of G-d. So the verse reads - "And Abraham saw G-d from a distance ..." [You could push this one stage further and take the word-play in Hebrew of 'seeing' and 'fearing' -- giving: "And Abraham feared G-d from a distance"..]
Abraham thought he was close to G-d, but actually he was far from G-d.
That is why he, Abraham, could have imagined that G-d really wanted him to sacrifice Isaac.
Our world is full of people who are far from G-d, but think that they are close to G-d, and therefore believe that G-d wants them to do appalling things. In the case of Abraham, G-d stopped him in time. He was never intended to kill Isaac, and G-d wanted to know whether he, Abraham, really understood the quality of mercy of G-d or not. [Edited:] Abraham passed the test on one basic level and was complimented on that by the angel. Yet could he have also not earned praise by refusing the command? Could there not have been two ways of passing the test - at two different levels?
Second thought, to which I have no answer:
- On Rosh Hashanah, we read of the two sons of Abraham, one of whom is exiled into the desert, and the other of whom is taken for sacrifice Har haMoriah (= the Bet Hamikdash).
- On Yom Hakippurim, we read of the two goats, one of whom is exiled into the desert, and the other of whom is taken for sacrifice in the Bet Hamikdash (= Har haMoriah).