I agree that maths will be a very hard A level if the basics are not secure, which is why a B grade might not be secure enough. However, it can be taken just for the AS which will be a qualification next year, and then can be dropped.
The subjects continued to A2 level will just be a full A level. So most of them will end up with 3 A levels and 1 AS level if you see what I mean. An AS in maths is higher than a gcse and will be recognised by employers, etc. even if not actually used to get on to a uni course.
HSM what a shame all those subject clash. If she doesn't take maths, is there anything else she is interested in, history, media studies, business studies, etc?
AntiqueSinger so sorry to hear what's happened but great results for your ds and he can definitely go on to do the A levels of his choice 
If you want to offload, relationships can be a very supportive and helpful place to do it. Name change if you want to keep it separate from your other posts. Take care x