Hi, I checked out the webpage at Murcia. They haven't put any info. about the grades necessary for admission, which is strange. You said you had spoken to them, and they didn't give you any satisfactory answer. I would try again, maybe to the Dean's office: decanato(dot)veterinaria(at)um(dot)es
One possibility is that they are now changing from Licenciatura to Grado, and there may be internal confusion. I would ask again.
As for the certificate, from what I know, I need the Apostille de Haya when I applied for the doctoral programme. But for Bachelor's degree it may be different. I don't know how they recognize the A level, here the "selectividad". The faculty i am working in now only admits foreign students that have sat for the "selectividad" here.
Sorry that I can't be of much help. But if you wish, I can write a message to Murcia to ask. I understand all the confusion and frustration, and the oppressive amount of bureacracy, I went through the same when I applied!