Everything @doesanybodyhaveamap has said is good advice. There is a thing called UCAS Extra which you can use if you are rejected by all the universities you apply to, to add another few choices later in the year. I suppose, theoretically, at that point you could change direction and apply for academic degrees and rewrite your 3 answers accordingly. But you would need to do pretty early applications to all your performing arts UCAS choices and hope they all made decisions quite quickly. GSA tend to give you a yes, a no or a waiting list place reasonably quickly, but who knows how long you might sit on a waiting list? Central, by contrast, waited months to give my DS any news at all after Finals, and when the no finally came in May (after Finals in Jan) it was explained that he was a reserve - but it would have been too late at that point to make any new applications via UCAS. As suggested above, for an academic degree, at that point you'd be better off looking in Clearing. There were loads of great degrees on offer this year. Places like Arts Ed and EDA do offer throughout the year but they also keep lots on waiting lists, and Mountview do big finals in about late April or May so that won't really help you.
I would also say though that if your DD's strong preference is performing, then in all honesty, applying for academic degrees as a back-up probably isn't going to fill her with enthusiasm. These kids tend to know what they want! Is she genuinely torn over which direction to go in, or is she paying lip service to academic degrees because she doesn't want to seem arrogant or presumptuous about getting a MT place? Maybe you could agree to let the first year of applying just be all about performing arts. Having seen your DD on stage I honestly think she'll get a place and you won't have this quandary. However, in the unlikely event of her not getting an MT degree offer, she could also apply for a couple of Foundations to keep her skills going and brush up on any if needed, have a Gap Year doing a Foundation and then if that doesn't work to get her onto an MT degree, start looking at a change of direction after that. There's no absolute rush to go at 18, and if she took an academic degree just to have something, when her heart wasn't really in it, she might forever wonder if maybe one more try might not have secured her the performing arts degree she really wanted. It's a lot of work on top of A levels, finding and learning songs and monologues, making self-tapes, travelling to auditions etc. I think I would be inclined to let MT be the focus for the first year of applying at least, and then if that doesn't work out, you can look at alternative options either in Clearing or during a gap year. Just my opinion, but I think academic applications might just muddy the waters unnecessarily unless she would genuinely be happy with an academic degree as an outcome next year.