This is a really interesting spin on the idea. Not sure that the story is that deep
but I like the idea that she was perhaps trying to facilitate. I just think that there are some interesting ideas in the story and some real emotions that everyone seems to ignore to focus on NTR or not
Wehn Deb says at one point that she'll only ever be the "other woman" I think that is what Marie was trying to avoid for Millie.
Look the reality is alot of the arguing happening is done from the point of view of the game universe being a real universe so it has to function by some form of logic. The truth is that whoever made this game made a shoddy story full of plot holes because they obviously didn't have a concise idea of how their story was supposed to develop and happen from the start, they likely had an idea for the background and the starting point of the story, but no idea how it would work effectively in the long term, all they knew is there is the gimmick of 'separating parent and child to facilitate a incest relationship' that you find in a ton of games with incest, so they rolled with that without really having a precise idea of how things got to that point.
Now they decided that in addition to that they needed some drama & mystery and boom you've got the beginning of the story and the background for it as of the start. Problem is when it came down to making it work they went around the bend trying to justify everything while leaving the female characters clean, because it was also meant to be a harem game, and that is also a gimmick for harem game, to make all females somewhat descent to justify you being able to bang them.
The combination of trying to justify the incest in addition to trying to justify the harem, equate to a minefield where no female can be held responsible for anything wrong, while something has to definitely have went wrong to separate the parent and child. So the Devs have create a story that doesn't hold together, but rather than acknowledging the simple fact most people here are trying to argue their way from a in universe point of view with a sight toward reality, to justify all the non-sense we are being fed, but the simple truth is that the story being shoddy is the reason for all those situation, not any form of thought process from the characters.
I find it odd that EVERY woman in this game is bisexual, there's a lesbian free-for-all constantly where all women crosspollinate (if that was a thing) at the drop of a hat. Deb is an annoying twat that seems hellbent on fucking things up for her brother, being abusive without any cause and/or reason. Most of the writing makes me dislike every character in the game, Astrid being one of the least annoying sofar. It doesn't make sense, why would the MC take so much abuse all the time like the first half of the game where he had a damn letter from his former wife taking some heat off of himself from Milly ?
Its a harem story, what do you expect? Obviously every woman are going to be bisexual, its the only way a harem would work effectively if the female members can also be in a relationship with each others. Now as for most characters sucking, you are perfectly right, pretty much everyone short of Astrid and Kumiko annoy me too, but Astrid is sorta... boring... so frankly the only one I am interested in at this point is Kumiko.
As for why the MC is taking so much abuse? Because the story is a mess and by virtue of the harem the females can't be held accountable for anything, so obviously no matter how shitty things turns out the females have to be effectively 'descent' the problem is someone need to be blamed, so obviously the MC is the one to get the blame tossed on as a facilitator for the story. Like I mentioned previously someone should have taken the fall, the problem is most candidates are female and by virtue of the harem can't be blamed, the Devs obviously weren't imaginative enough to find another man to take the fall, so obviously it had to be the MC.
Also it browbeat the MC in taking the path intended by the Devs, the Devs want a harem made out of pretty much every named females in the story, so you can't actually distance yourself from any, after how Milly is bitching at you in the first half I decided to restart and forgo any romantic relationship with her but no matter what the game has been trying desperately to shove it down my throat, so you can be damn sure it will do the same with everyone else. I might be able to avoid it but they'll damn sure keep every female at hand so that if I change idea at any point they can toss them at me in a snap.