This is Mumsnet so anything involving a man will lead to comments of LTB. Also a lot of people projecting.
Personally I think she sounds like a spoiled manipulative brat,and given she’s almost an adult she’s close to becoming a bitch in the making.
And the stepmother vs stepson would result in different posts precisely because the stepson would be male
Teenagers can be horrible creatures. Even most parents lose it with their teenagers sometimes, but the difference is that if parents do it the teenagers can’t strop off or demand the one parent end the relationship.
I have a seventeen year old who doesn’t pull his weight. In many areas I pick my battles but in others there is no ccompromise. E.g. I asked him to bring all rubbish down before the next morning as it was bin day, I had been asking him all week so this particular day I told him that if he didn’t bring it down then i would be waking him up at 7 AM to do it,and not to think that I wouldn’t because I absolutely would, and set my alarm accordingly. I absolutely would have done it, but aaaah surprise, the rubbish was brought down the night before......
Last year I spent six weeks in hospital, and while I was there my parents stayed in my house with my partner there at weekends so they could go home. My DS started his GCSE’s the day I went into hospital,I spent two weeks in ICU, had a full crash,a cardiac arrest and heart surgery and a further four weeks on the wards.So I think it’s fair to say that stress was fairly evident on all sides.
Anyway one night my DS and my mum had an absolutely massive falling out which resulted in my dad going in and shouting at my DS. He immediately texted me afterwards because he felt so guilty. He apologised to DS but DS was having none of it. I spoke to DS and while I made allowance for the fact that he was stressed over both his exams and me,that the way he had spoken to people wasn’t on and that he had as much blame as they did.
Things are rarely black and white.
I would call her bluff at this stage. If she wants to move back to her dad’s then i would let her. But I would do so on the understanding that your partner will no longer be funding her university since she obviously doesn’t want him to, because by saying you need to end the relationship that is effectively what she’s saying. She can’t pick and choose which bits she wants. Either she accepts that you’re together,and given she’s now left home she has no right to demand you split up with him, or she demands you split, which means she wants nothing from him including his money.