I sense misunderstanding and a need for clarification.
You ask how can a Sorcerer cast Time Stop in Chapter 3. Let it be known that a solo Sorcerer can easily enter Chapter 4 at level 25+. Solo Sorcerers gain XP like nobody's business, because people die, lots of people die.
A Mage loses out in this aspect, and suffers the IWD-HoF syndrome. All the spell slots in the world, and not enough good spells to fill them up. You will be hard pressed to have a good and varied selection in your level 7-9 spells before entering Chapter 6.
Sorcerers can meet Dragons, Liches, Mind Flayers, Golems, etc... and use Time Stop + PRATI to rain Hellfire and Brimstone upon their unsuspecting heads. Men will die, wives will weep and children shall huddle under their beds when a Sorcerer really gets going in the game.
A Mage can emulate the Sorcerer to a certain extent, but where a Mage is a Nuclear Bomb, a Sorcerer is a Hydrogen Bomb.
A Sorcerer
is more versatile in the heat of the battle. Understand this, if your Mage only memorised one Stoneskin, you only get one Stoneskin during the battle. A Sorcerer can still have a second, then a third, maybe even a fourth for a lark.
Trying to argue that a Mage is more versatile than a Sorcerer during a battle is futile, because it is simply not true. A simple test to demostrate this is to ask yourself, how many Mordenkainen Swords can either Mage or Sorcerer cast without the aid of Spell Trap, Wish or Resting?
The answer will be rather disappointing to Mage users.
