I really don't want this to be that. I hope he left because he realised she's abusive, not because his plan was…
you're right, he found out about his father gradually. he was so done with his mom after she treated him so bad before he left the house. and im glad mijoo was with him after that to kinda help him "cure" a bit. not really though when he found out about the death of his dad :( i really hope kangho and mijoo get the happy ending out of this :(
kangho basically treated his mom just how she treated him before. harshly but for their own good. only kangho's plan was even more dangerous for them, so he had to "abandon" his mom. i'm sure he was already prepared for the accident caused by oh tae soo's daughter.