ok so

Wait till never figured out why it was a bad thing (⊙…⊙ ) why is it bad if he knows?

I think because he will be aware that her kingdom wants to take her back. They treat her like an irriplacible saint or something because of her ridiculous amount of power and there are people like her apprentice that would try to get her back because of their personal affections. She also fears her country finding out about her and trying to take her back.

His daughter is somehow an arch mage from another country, who is being searched for by that country, and who may cause a war if it gets out. On top of that it is also unclear how she became his baby in the first place, and she is supposed to be that promised third star and let's be real here if my child had the mind of an adult I would be pretty disturbed

Disturbed the baby didn't tell you and keeping a hidden personality, or disturbed by it like it's a birth defect??? Or do you mean disturbed that it's even possible a baby is born with an adult mind??? Sorry, I know that wasn't the point of your comment, but I'm just curious.

the last part of this was my actual reasoning

Disturbed in the sense, what happened to the actual princess was there even one to begin with what whould have been if this person wasn't in my child, is everything they done a lie/act, is this person even my child (since the mind is that of a stranger).
I've picked the argument up from The beginning after the end (slight spoilers ahead for that one)
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it's a part of the plot(not in the manga yet only in the novel) when the main tells his parents

Oh, cool cool. Good point!

I always wondered about that at the beginning but then I read so many of these I just forgot about it. For some of them, they gradually get their consciousness a few moments, days, or weeks after they're born as if their body had to get over of the shock of just being born or their brain needed to develop more to have coherent thoughts. Implying they were really just reincarnated. Also, in real life when people suddenly get a big chunk of their memory back, they forget everything they did while they didn't remember, like a switch, so I started assuming they are the real body's host unless clearly stated otherwise in the story. I can only remember two or three where the main character really was just a visitor to the host body, and the body eventually started to reject their souls in both cases.