That's interesting. This article contradicts your view claiming that often times the only way to make a perp finally give up the location of a victim's body is precisely by having him face a capital murder charge. In other words, it seems prosecutors find that certain defendants will only give up the location of a body when they have been charged with a death penalty case, and they then use telling authorities where the body is to bargain themselves down from a potential death penalty sentence. http://www.huffingtonpost.com/diane-dimond/no-body-murder-cases_b_137791.html
I'm not sure it is a contradiction. If the Feds had evidence to bring a murder charge, they would still need to get AG approval. I would agree that the death penalty would persuade many to give up information. Since murder hasn't been charged, I don't think they have the evidence to support it. IMHO, I believe they may have felt compelled to arrest based on circumstantial evidence and his comments on the tape in order to prevent another kidnapping.
Given what is publicly known, I see the kidnapping charge as weak. If there is no case for kidnapping, then the Feds don't have jurisdiction and there is no death penalty leverage. That no murder charges have been brought indicates to me that they either don't have the evidence to bring one or what they have is weak and the AG is taking his time as a result.