You are absolutelly right! (though, I think we still can blame Celine - she did mess up Rumi)
I can call all this situation "Celine's Wormhole", because the longer you think about it, the more complicated it becomes.
Realistically, the most sensible thing a hunter must have done in her place is outright kill Rumi while she still was an infant. There are little to no chances for this to end in a good way.
The next sensible thing would be not to teach Rumi how to fight and keep her under constant surveillance.
Not only Celine taught Rumi how to fight, she also made her the leader of the next Hunter's generation, and then she gave her the full rein over both Hunters and Huntr/x. Not only Celine is not controlling Rumi's every step, she is not even checking up on her.
We know Celine cares for Honmoon and Hunter's duties. The amount of trust she put in Rumi is astonishing. She was absolutely sure that Rumi will never betray them. So, the instilled rule to hide the patterns was only to protect Rumi - primarily from Gwi-ma, but from Mira and Zoey also. It was never a mean to subdue Rumi herself.
Your "Celine would actually be *right* that Rumi isn't a demon" is the start of this "Celine's Wormhole", I think...
Dangsan Namu is the place where late Hunter's buried. And this is the place were the new generation of Hunters being trained. That's particularly because this place is protected from demons, as in "demons can't go near it".
And, I think, that would be the first place where Celine brought Rumi to. And was "allowed" to enter it. By this fact alone, Celine could concluded that Rumi isn't a demon. But the patterns are still there...
(I don't know how to interpret almost every one of Celine's lines)
This is me reaching really hard, but this line in particular "This is why we have to hide it," telling me that Celine already saw those ripples on Honmoon. What if she did? What if baby Rumi's cries for Mi-yeong already caused those ripples 22 years ago?
So, Celine had on her hands Mi-yeong's baby, who was "allowed" near Dangsan Namu, but has patterns, and with the voice that can rip the Honmoon apart...
She is not human, she is not demon. How exactly should you raise a child, when you are not sure what this child even is?.. What "good" choice even is by that point?
Add the fact that Mi-yeong death should have ended up a national news. And Celine didn't have the oppurtunity to grieve in peace.
Celine had to manage her public persona. Had to manage the brand. Had to laid the foundtaion for Huntr'x debut.
All the while, protecting the Honoom and killing demons.
And, don't forget, that she also had to sing (doing concerts, recording solo albums) in order to feed the Honmoon.
She also had to find, sowehow, new Hunter's generation and train them.
AND to be a good mother on top of all of that?!?
Celine was awake for 40 hours a day. And she had to do it everyday for 20 years.
By the end of the day, Rumi is a well adjusted person, a good person, and a really good Hunter that didn't succumb to her demon side even at her lowest. That's too Celine's doing.