It's unsettling but understandable. It's just normal in that world so he doesn't think he is doing anything wrong. Human morals depend on the collective sense of what is "common". If some other worlders come to our world today and saw how calm and colected government officials declare wars and justify violence, I'm sure they'd be surprised.
In that world, it seems that enslavement is the norm. Nobles would have slaves just the same way they have maids and butlers so it's nothing out of the ordinary.
But yeah it's weird for someone from the 21st century so that's why the MC is determined to do something about it
No, in the original story the dad died the day she got her power (so the day the manga start) and her mother is never there until she die (right now we don't know if she'll die but it's a fact that she is neglecting her daughter, whether she is doing it on purpose or not).
So in the original story, there was no adult with higher authority than her during the time she tricked him to make a slave contract.
(The difference is that the slave contract forced him to obey any of her orders, she used it to make him kill his mother)
how can her father talk about a slave contract with a smile