To me, the mother's involvement just seems unnecessary and weakens the story with the sisters. I hate when you're trying to seduce one girl, and things seem to be progressing, but then suddenly another love interest interjects to move things along. Now that I think about it, Parental Love did this, where the MC's sister involved herself with helping the MC have sex with his daughters when they were already obviously interested in him romantically. There's been other games that have done this as well, so it's just not a Luxee thing. But the experience left me annoyed at the sister for most of the game.
I'll give points to Luxee for doing something different with the mother. Except I'll take points away because he deviated from the type of stuff that most people like, and moved it into territory where it just hurts the stories with the sisters. And I just can't get interested in the mother at all because of all of this. I don't really get the idea she's even a real interest, but rather just there as a (unnecessary) plot device. I'd much prefer she function as an initially reluctant love interest and thorn in the side of the MC as he tries to seduce his sisters. Having her approval from the beginning just gives the feeling you're on easy mode now and bypasses what could be a lot of interesting tension and story in the game. But this game seems to be all about skipping over development for most of the love interests.
I'll also agree with your earlier assessment that anything non-Nea is a 3/5, although that might be generous. I either haven't played through the Nea stuff or fast-forwarded through it, but I don't dispute it's a 5/5 based on it clearly being Luxee's passion to tell her story. But he should be aiming for 5/5 for everything here, or if he can't hit that, at least aim for 4/5 for everyone instead of this jarring drop in content and quality for anyone other than Nea. That's how we end up with Nea fans seeing one game focused on romancing a trans girl that is perfect for them, and others seeing a mediocre harem/incest/loli game.
I absolutely despise Parental Love. It caused me to hate several troupes and concepts found in other adult games because of that exact scene. I also hate games when relationships start before the game begins, especially incest. I also disliked slutty characters for a while, but I outgrew this one for the most part I think.
Back on topic though. I don't remember too much about the game outside of Nea. I only remember that I hated the family content and why. That said, I don't believe I hated the mom, just the story she caused. If this game rebooted or a copycat gets made, I would still want her as a LI. However, the manipulation absolutely needs to die. At the very least, she takes the role of the older sister from My New Family where AFTER the relationship starts, she plays the wingman to help escalate relationships that have already progressed to a specific point.
As for Nea, I've always claimed that characters are the most important part of AVNs. A great character is enough to make someone love even the worst game. I entirely believe that Nea is that type of character. I understand why she would be polarizing for many, but when you create a character that's so loved by a portion of your audience, I think that's a character you should prioritize over others.
That's not to say that you should ignore other characters, that I agree with. I only played this game because it was a harem. While I walked out a newfound fan of trans content, not everyone will. That said, I'm not sure how much Nea actually influenced the quality of the game. I could be completely wrong, but I think Nea was a later addition to the game. Meaning, that a lot of the crap you and I despise about the family dynamic was already in place before Nea was. Take this with a grain of salt.