OP from the posts on here, people are posting in good faith and trying to give you the best advice they can in respect of your DD's choices.
The evidence is clear. GCSE results are a very good indication of performance at A Level.
To reiterate, at a grade 7 (a very respectable grade) the overwhelming likelihood is a C/D at A Level.
You say GCSE grade is not everything and yes, there will always be outlier's but the chances of you DD (especially when she's not even a solid 7 and is being predicted a 6 and has struggled with her MH) being one of them is negligible.
I don't say this to be unkind. I genuinely wish you and your DD well, which is exactly why I would steer you away from A Level maths.
It's also worth really reconsidering what another poster pointed out.
A grade 7 only equals roughly a 60% pass rate. The questions that get the 8/9 grades are exactly those that test the maths that is a gateway to A Level and your DD is not successfully answering those questions.
I think people see maths as an homogeneous subject but it's not. Some people can be great at statistics but fall flat when it comes to mechanics. To do well at A Level you have be good at all of them.
The best analogy I can think of is if you study history and one module is WWI and another Ancient Rome, even if the latter time period doesn't float your boat, the mindset and techniques to do well in one still apply to the other.
That just not true in maths because to get Uni level grades, you have to be as a minimum competent in all modules. Scoring A* in statistics won't offset D/E level performance in core/mechanics/pure.
Even with the best teachers and tutoring, in the circumstances you describe (and where you have already stated the pressure of taking 4 A Levels on the basis she could drop maths if it didn't work out would be too much pressure) I think you really need to reflect on what people have posted.
I've nothing to gain in strongly suggesting she pick a different A Level, especially when you've indicated she doesn't need maths to further her higher educational aspirations - rather I wish you both well and say kindly that A Levels are hard enough without introducing unnecessary pressure and risk.