This is hard to say, but I think if he is guilty of what would be the worst that could have happened, then decomposition would lessen the evidence perhaps?I mean, if it was an unfortunate accident, it would have been to his advantage to come forward sooner to confirm.
If he pleads not guilty, refuses to say a word, no body found, his chances are probably better that he is not convicted, particularly of Murder 1, depending on what forensic evidence is found. THough yes, it's possible, it's more difficult to get a murder conviction without the body. THe Alexis Murphy case did turn out that way, but there was a lot of evidence, and Randy Taylor talked ... a lot, and most of it proven outright as lies. If JM just stays quiet, lets his attorney do the talking, he can save some face with friends and family by later telling them his version of what happened which may not have certain details, and isn't put under scrutiny. Like he picked her up, they went out, they got cozy, she got upset and wanted out, so he let her out, and that was the last he saw of her, and then he finds out she went missing and as a black man in a southern town, a townie, being with a white UVA student, he panicked and was scared to death realizing he could have no proof if she didn't show up, and he was hoping she would.
Just a quick run down of a sample story.
A lot of times, it comes down to being able to stick to a story that those you care about can swallow and hang on to, not one that will pan out if you tell the authorities, because, frankly, it many not be the truth. There are loved ones of criminals who believe with all of their hearts what they've been told, some despite evidence to the contrary blaring in their faces, but some can have such stories without direct debuttals because the court, LE never got to hear the stories to debunk them, and so it can stand up privately. You can't do that if you say X, and LE disproves it flat out. The jury could choose to reject LE's, the state's evidence and version of what happened to the defense's suppositions and possibilities. Remember, the defendent is innocent until proven guilty, does not have to say a word, tell his story, do anything. Just let his attorney tear up the prosecution's theories, poke holes in the evidence, and not offer a thing in return. You get off, or a short dealt sentence and you tell everyone the "real story" that you knew would not be believed and that your attorney said just to keep under wraps.
That is also the case for some events that you can't prove even though it's the truth, and you just aren't sharp enough to take the questioning of a prosecutor tearing it to shreds, true or not.