German LE have said they have 90% of the evidence they need. And, also they believe they know CB murdered MM and how, but have no forensic evidence which is ideally what they would love to have.
Being there on the day might not "nail" him, but it builds the case. As I said in my previous post, he must have given an account of his movements around the time of the abduction when he was questioned back in 2013, if it can proven that account is false, then it's another damning indictment to add to the list.
German LE don't have any forensic evidence and aren't guaranteed to get any so they need to work every angle. They have other evidence we don't know about yet, in the absence of a body and a murder scene, these are the final pieces to the puzzle they want to be able to make their case.
German law works by putting all the evidence in front of a judge and they alone decide, whether they think that person is guilty or not. As the prosecutor, HCW knows what level of evidence would be required to get a conviction.