
Can anyone remind me why aiden had to leave 9 years ago? Or was it not specified the reason?

Because of his family, his dad or grandpa was brother to the king, so because of that they had to go hack to their territory as to not enter into conflict with the royal family, I think, I'm not sure, from what I remember that's what I understood, but he could have just sent a letter explaining what was happening before actually stop sending the letters, but he didn't, so he had no concern for her feelings which is why she was so hurt and with time stopped loving him and got over her live for him

Not to burst your bubbles, but there's still the matters with yeonwoo's bullies bye--

I think in the current state of yeonwoo, I would probably assume that he might be able to handle them now... of course there will be setbacks but I really think that yeonwoo willingly and able to help someone else for their personal matter is a strong sign that he is not as fragile as before, she will probably help haesol deal with their parents too (▰˘◡˘▰)
I hope Mikhail will only stay as older brother to her.