Walkthrough tips, Discussion of Campaigns [Heroes 5]

Regarding spells.

1. Can a hero only learn spells that are studied in the Tower of Magic from the 1st to the 5th circle? (not counting the special skill)

2. The first time I played, I defeated Haven with the Academy, and I had the Phoenix Summon spell learned when I built the 5th level Tower of Magic.
The second time I played, I defeated Inferno with the Academy, and in the 5th circle, instead of Phoenix Summon, I learned Divine Shield (and Phoenix Summon should have been in its place, judging by how the spells are displayed in the 5th level Tower of Magic when you visit it).

Question - Where did the Phoenix Summon spell go? The first time it was there, and the second time Divine Shield appeared instead: confused. Could it be that the spells learned change depending on the hero you choose when you start the game?