God she's so good. She knew so much about him and exactly how to pull at his heartstrings and make sure that he would lean in her direction no matter what would happen next. She cast herself as weak, pitiful, in need of protection, and that his family would disapprove because she can't have children and has already been married. Planting the seed that any objections or concerns about her are related to those, meaning that if they did catch on to the fact that she has something insane planned that involves the daughter, he'd mentally go back to this moment, when his proposal was a pledge to shield her and make her happy even if society has turned her away.
By casting herself as a sad, lonely woman who isn't sure that she even deserves to be happy, she's made herself a character that accusations against feel cruel and unjust. Because she believes them, right? She tried to stop him from marrying her because she thought she couldn't make him happy, right? How could she have been planning all of this from the beginning, how could she be manipulating him, how could she be anything other than a sweet woman who deserves to be loved if she tried to stop him, right? Accusations against her now engage his protective instinct, which we know and she knows is very, very strong. He's a good guy, after all. We know that.
I think some new information was revealed here, though. I had always thought they were the same age, but she's talking here like she's older. That, or she's casting herself as young too, but her life is already over while his isn't. She specifically says that he's still young, which usually is said to contrast yourself against the person you're talking to. I don't know how much it matters, because I think they're still basically the same age, but it's still a bit of information that we've never had before.
The problem with your interpretation was that she picked him from a list that she had already gone down and crossed out several names. This is revealed in a chapter where we see belongings, such as paper and clothing, being burned and disposed of, indicating that they are changing things up and moving on from a previous attempt at doing... whatever it is they're doing. She also artificially creates the circumstances that bring them together; deliberately leaving behind her notebook and knowing that he'll try to give it back to her.
The reason people are talking about something occult is because over time we've come to understand the major goal of their plan, which is for him to get the daughter pregnant and have a male child, but not fully why or what its meant to accomplish, but it seems to be to bring back the daughter's father from what we know. How do we know that? Chapter 21 and 22, with some context in 21.5.
So in chapter 21, Hiroto(our boy) and Touko(the daughter) go out into the ocean to play, and there's a moment when a little kid mistakes Hiroto for his dad. Which makes Touko start talking about how she wouldn't mind being an older sister. While our guy starts dreaming of a future he had decided to give up(we see why THIS chapter,) she bursts his bubble by saying that her mom's body probably couldn't do it. But then goes: hey, you always have me. Now if you literally ignore literally everything about what's being said, you can read the full conversation as her saying "well I'm your kid!" But if you have eyeballs, it's absolutely impossible to ignore that she's straight up saying "hey I can have a child you know." After saying that her mom's body is too frail, she says "if that's the case," while running a hand from her chest down to her literal vagina, "I will be there for you." Then, she goes "because, even if we're not related by blood, I will still be papa's daughter!"
SO. While that's a bombshell, the end there feels like a 'oh, ok' type dodge. But I don't think so. I think the use of 'papa' there is deliberate, because of what happens in chapter 22. In that, Touko and her mom are having a meal in a restaurant. Her mom says that Touko's face looks just like "her papa's." That if she was a boy, she would've looked just like him by now. They then talk about how Hiroto wants to have kids. Notably saying his name and not calling him papa or dad or anything else, and saying that their visit to his family was a good idea. If you've been reading the comic, them saying their visit was a good idea at this point is fucking wild because it's been nothing but drama. But, hey... almost like they planned that from the beginning, huh? Touko says that she can't wait, indicating she's just super excited to have a new child enter the family. Or be pregnant. One of them. Both of them?
Now here's the key: they get ice cream, push it over to the empty seat, and Touko says "just wait a bit more, papa." Considering that 'papa' has, in this conversation, referred entirely to the unnamed father of Touko, the implication here is clear: just wait a bit, papa, you'll be here soon, because we're working on a way to get you back. Because if it was Hiroto they were talking about, they wouldn't need to wait. He's already part of the family. He would've been with them if they invited him. The beginning of the chapter is him realizing that he's becoming distant from his old friends and really only has close ties with his family, by which he means his wife and step-daughter. So papa only makes sense if they're referring to the unnamed father of Touko. Which is why I think that when Touko says she's still papa's daughter, she's referring to THAT papa; because part of the plan has been for her to have a kid the whole time. Is that a stretch? Only in the regard to seeing a connection between that one time she says papa and the conversation in the next chapter. But the whole plan, the whole idea... that's out there, man. That's said.
Now in chapter 21.5, we see an interesting bit of context to who Touko's father is. When Miyako(the wife) is sitting on the beach and watching her daughter and husband walk out into the ocean to play, she remembers something. Her, clearly much younger and maybe not even an adult, covering herself up and being afraid to go out into the water. But a dark figure who we can't see fully holds out his hand to her, tells her to come along, and she eventually does. She recalls this fondly, while watching her husband and daughter go out into the water.
What could that mean? Fully, we don't know. But I believe it speaks to the origins of Touko; her father was at least some sort of guardian role over Miyako, because that's what the figure is acting like. She remembers her time with him and the pregnancy she had with him fondly, and if it happened when she was too young to actually have a kid properly, which math CAN indicate(she's in her 30s, Touko's 16... depending on how far in she is...) then that may be why she can't have kids anymore. Either way, it speaks to the idea of 'papa' being a bit of a 'papa' figure for Miyako, too.
Thus, this whole thing almost seems cyclical, and the title, which means "welcome back, papa," begins to have a much more sinister implication.
