I think it totally depends on the DC and what they are aiming for. GCSE grades can absolutely count - even post A levels. And if there. have been mistakes or marks missed then of course a review of marking is appropriate.
Sadly the playing field will never ever be level, no matter what the exam boards do. My eldest has SEN, really struggled with A levels, we paid to support college work with tutors. This really helped, and he ended up with decent grades. Another student with SEN really struggling with A levels might have parents who can't afford to supplement with tutoring, and therefore not manage decent grades. That feels unfair. But I don't see how you avoid parents doing their best to support their DC in whatever way they can.
At the upper end of the grading, though, I am not sure whether even Oxbridge or the super competitive courses like medicine, vetmed, dentistry etc do really distinguish between grades 8 and 9. As a friend once pointed out to me, in Wales they have no 9 equivalent (still have the A star, A, B grading there) so technically a university has to consider 8/9 as comparable - otherwise they'd be discriminating against a Welsh student for whom it is literally impossible to get the equivalent of a 9.
How are the various DC getting on at sixth form? DS has a maths test this week. Seems to be enjoying all subjects, although I'd say he is very much doing maths because it will support his physics and chemistry, rather than for pure enjoyment of the maths. Science is definitely his 'thing' and he's currently thinking of natural science for uni. Early days though!