What’s the highest character level in Torchlight

Yes, replaying the story is the only way to earn the acheivements.

I was in this situation myself, but I used the special function to "retire" my character, so at least I had bonuses with my new one.

If I remember well experience gained depends on monster levels and does not scale with your level. You are right, the experience that you get from killing mini-bosses is very high compared to that acquired from killing normal monsters.

There is also a type of fish that increase your XP gain by 20% for 300 seconds. Remember to eat one of that before big fights.