I'd say right now there is enough evidence to prove Jacques committed the murder based only on what is known publicly: detailed plan to abduct and rape her, testimony placing Brooke last alive in his home, planting false evidence, history of sex crimes. I suspect they already also have the cause of death as well as statements from Juvenile 2, more statements from AR and possibly more statements from Gagnon. And if he had help, the other accomplices would be likewise charged with murder whether they actively participated or not...under the joint venture rule the state doesn't even have to know who committed the murder to get a conviction, but of course it helps.
I'd also say that the FBI investigation spanning at least 3 states is evidence that they are leaving no stone unturned in getting evidence against Jacques and Gagnon.
What's the rush? Jacques isn't going anywhere and neither is Gagnon. My feeling is they are going to wait until they have ALL the charges ready to go, rather than doing it piecemeal.