I agree that the teacher shouldn’t swear - and that concern should be passed to the head, with evidence.
But, I don’t think he should have to call parents outside of parents evening.
He might eat and drink in class because he works throughout his lunch break.
Unfortunately, class sizes are too big. Your child is one of hundreds of children he teaches. To be brutally honest - he’ll be targeting the ones who show the potential to pass an exam one day, and he’ll focus on pushing them to get there.
Is she receiving interventions for her sudden dip? If not, then she is probably working at the level expected for her. If she is suddenly working below her target level, then she should be receiving an intervention to get her back on track.
If she’d been in top sets until now, and had slipped, I would understand your concerns about this individual teacher being the culprit - but it sounds like she’s always been a low attaining pupil, from you’ve said. Did you ever ask primary school for her data each parents evening?
Seemingly, she enjoyed year 7, but that might have been down to a friendlier, less challenging teacher, not necessarily a better teacher. That teacher might have pitched the classwork and tests lower, to boost apparent results.
It sounds to me like the recent 50 question test was pitched way too high for that set, as none of them did well.
If you’re hoping she’s going to get a maths GCSE, and if that is appropriate for her level, then I would absolutely get a tutor for her, if I were you.