For all we know, the murder charge may be dropped at any time until the trial. The abduction alone is enough to try him on and keep him in custody.
I was surprised that they charged him with abduction and murder at the same time. They didn't need to do that. They could have just charged him with abduction, as you say that would have kept him in custody. It would also have allowed them to gather more evidence of murder and to continue to question him about the new evidence. They didn't do that which has to point to them being pretty confident in the evidence they already have.