I like the last idea the most. One thing though, it might require more Dice to use during character creation to allow for more starting spells.
But, limiting a caster type's starting Dice would work quite well for the 'typical' spell caster type. Y'know the kind; physically weak, no martial skills but are quite potent even from the start.
Also, a thought about the penalty at the beginning...
Maybe instead of forbidding the character to learn any other magic, you could make spell from other lists cost double for purchasing and improving.