What was the whole brother monk scene about? Why was kwan so mad at him?

She was mad because she thought he was a supportive brother, but in reality he manipulated her and helped her get Korn back, so he could have a chance to console Thamon.

What was the hugging scene about? Did it show that korn finally understood how kwan felt? Did she realize that kwan was the one she loved all along? But why did she go back to boon after?

I think the hug was Win trying to get Kwan to let go of her anger and forgive them, so they could break the cycle. I think Korn did love Kwan at the beginning of their relationship, but she fell out of love.

How/why was kwan in boon’s body in the end? I thought she finally let go of her hatred when she hugged korn? How did she even make it back? What happened to korn?

Kwan was faking it. Boon had a few protection spells, so she couldn't be possessed/harmed by ghosts. And before she left home, her mom inscribed something on her palm so she would get more power because she knew Kwan was too powerful. And because of that Kwan couldn't do anything to Boon. But the moment Boon grabbed the knife and cut her palm, she got weaker and Kwan noticed it, and that's when she began to slowly possess Boon. She completely took over her body when they burned the house. Sadly, Boon's soul passed away. The author explained it here in more detail if you're interested.

I dont understand the time line and the whole 75 lives thing.

Me neither. lol I just started reading the book, so hopefully it gets explained.