How can I obtain Steam achievements in Torchlight after "finishing" the main story?
I finished the main story of Torchlight way before Steam introduced achievements. Now I have my Level 50 character and am doing quests.
Is the only way to get some of the achievements (such as "Over the Brink", etc.) for events occurring in the main story to replay it?
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.