
Alright! (spoiler alert)
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ok so in the novel Keira kinda forgives him,but not really. It's more of a "I'm going to forgive you for myself and we aren't going to have a relationship" type of forgiveness. She and the other characters in the book leave him and start their own kind of family.
The father's actions are purely tied to trying to remain neutral. He knew that Keira was the real daughter (at the very last moment unfortunately) but still went through with executing her because he didn't want to take a side and not look neutral. Also the little brother and grandfather were trying really hard to stop the execution but unfortunately they couldn't.
The father decided to turn back time because the fake daughter was pretty much the devil and destroyed the kingdom, but he's forgotten in this timeline.
The webtoon might deviate from this, because the novel definitely didn't have a romantic aspect to it from what I've read.. but it should mostly stay the same.

I just checked where I get the novel spoilers from:
https://forum.novelupdates.com/threads/actually-i-was-the-real-one.117635/page-1
and unfortunately it seems like I was half wrong, this is a new update and I hadn't checked the thread in a while.
It seems that the author added an extra final chapter where Keira and everyone else comes back and forgives the father and loves him. Fucking insane, which is why reviews for this novel ended up being low apparently. I have no clue why the author decided to do this.
Sorry about this guys :/
LMAOO SEEING THE DAD SUFFER IS SUCH A SEROTONIN BOOST!! IDGAF IF HE GETS A “REDEMPTION” ARC HE DOESNT DESERVE ANYTHING!! LIKE HE WASNT A GOOD FATHER EVER?? HE WASNT A GOOD FATHER AT THE START AND HE BECAME WORSE WHEN THE OTHER BITCH CAME SO IDC IDC!! I HOPE SHE NEVER FORGIVES HIMMM