I'm not sure they'll be charged with capital murder. It kind of depends on what evidence is gathered. Simply murdering someone in Texas is not sufficient to get the death penalty. It all depends on what really happened.
But, if I had to guess, I'd say you're right.
Since the autopsy indicated no sexual assault and the cops said robbery was not a motive, I'm kind of thinking that at some point in the evening, she may have indicated she needed or wanted to go home and they detained her.
We don't know who moved her truck from the coffee shop parking lot to the nearby residential area, but it wasn't Ashton. It was a male, who came back later, after she had left in the white pickup with two other males.
Whether or not she gave them the keys to move it or whether one of them moved it so no one would be suspicious of her disappearance, we don't yet know. Chances are, if she were not incapacitated in some way, she would've come and moved her truck herself.
If she were forcibly detained at a later point, even after going willingly with them at first, that is still kidnapping. Kidnapping would be an enhancement of the murder charge and would qualify as capital murder.