No matter how many times I read this, I'm a bit confused with previous emperor and FL fath...

Not the previous emperor, the previous crown prince (he died before ever getting to the throne). It was Duke Retentor (who was also his maternal uncle) who raised him like that. Duke Retentor raised both the ML's father and later on the ML, and both grew up as crazy people.

Duke Retentor raised them both to be power-hungry enough to act like bloodthirsty killers.

Just to clarify, the previous blond long haired crown prince was just obsessed with FL's father the one she gave a pain reliaver herbs? Just an obsession or are they in relationship?

Yep. They exchanged love letters using fake names, at the very least (it was stated the "Esther Penselors" in the letter of chapter 15 was code name for "Hessel Orpenster", the name of the ML's dad, the former Crown Prince). In chapter 17, there is a flashback where the ML's father told the child ML how to monopolize a love interest, and he was talking about what he did to get the person he himself wanted, which we now learn was the FL's adoptive father. And the FL's adoptive father was the only one the ML's father didn't want to kill (he was the only one he tried to save immediately after stabbing him, which he did only bc the FL's father told him he knew all the truth... and the Crown Prince would have never hurt him otherwise)

The revelation that the "Esther Penselors" (in the letter of chapter 15) was code name for "Hessel Orpenster", the name of the ML's dad, comes in chapter 24.

And the more telling clue: chapter 20, where we see in a flashback that the crown prince even interfered the relationship between the FL's adoptive father and his fiancéé at the time, Leona (who later was forced to marry the crown prince himself, and gave birth to the ML, so she ended up being the ML's biological mother).

Glad I'm not the only one

Tbh, I also had to read it like 5 times to get it