Being a "focus" of an investigation simply means they are focusing their attention on him as the perp--ie they are investigating him as being the perp. Being a "person on interest" means they have reason to believe he may be involved. Pretty much the same thing. I think it's interesting the LE is applying Judge Danielson's gag order to Amber's case--a judge can only issue a gag order in a particular case--otherwise the order wouldn't pass constitutional muster as it would be a prior restraint of free speech. That makes me think LE has definite evidence that JAG is involved.
If the GAG order only involves Chelsea King case, why would LE apply it to Amber's case, regardless of if they believe he was involved or not? Since the LE is calling him a focus of investigation, then I don't think it's in dispute that they think he was involved. The question is, as to what evidence do they have if any.