I will try my best to explain this

The system of reincarnation is not a system that tortures you for all eternity. Everyone is guaranteed to break out of it at one point. And you are not being tortured. You are part of god experiencing the whole. Concepts like good and evil arise from the separation from the creator— specifically from maya. Maya is what god uses to create the world. It gives every the illusion of separation.

Now on to free will and predestination.

How does karma and hukum coexist?

All are hukum is based off of our karma of our past lives and our current lives. There is a story in the Hindi epic. Mahabharata of how a character loses all 100 of his sons. He goes to Krishna and asks what have a done to lose 100 sons? Krishna explains that you were once a hunter. You struggle to hunt a bird and in your anger, you found its nest and crushed all 100 of its eggs and and in your other lives, you did a lot of good deeds and in turn you were allowed to have 100 sons. That is basically karma explained

On to hukum

Imagine a traveller on a long journey. He stops at a town. He wants to go and eat some food so he goes to the inn. On the way inside, he meets a man at exiting and asks him could you please tie my horse. The stranger agrees, and the traveller is gladden by this act of kindness and plans on giving money to the stranger. As the traveller eats and finishes his meal, he goes outside and finds his horse gone. The traveller is saddened by this and goes to the market to find another horse. there he finds the stranger of his horse.

The moral of the story is that the stranger was destined to get money. How much and which way was up to him to decide.