I hope that continued digging by both the media and BPD will eventually turn up iron clad physical proof that he couldn't have done it and then ignore him.
But letting him get away with this stunt and leaving the matter hanging as possible but unprovable is just begging for continued posturing on the part of JMK.
Is there iron clad physical proof that *I* couldn't have killed her? Disproving the negative is always a difficult task.
Your benchmark should be "what can the DA prove in court" and, after establishing what can be proven, "is there enough proof that a reasonable jury would convict?"
If Karr managed to kill her and waltz away without leaving any witnesses, footprints, fingerprints or DNA then he probably gets away with committing the crime. You almost can't LUCK your way into leaving no evidence, particularly in a violent struggle.
If this was an intruder, he would had to have been such a pro as to wipe down all sorts of items used during the crime, vacuumed up all the hair and skin cells shed, altered their handwriting so as not to match the ransom note, etc and vanish all without being detected. MAYBE he can do that if he kept gloves and a ski mask on at all times but, otherwise, most people just aren't capable of being so neat.
Which leads me back to the likelihood that either someone already familiar to the Ramseys (and thus has an alibi for leaving trace evidence in the home) did it or that the Ramseys covered up the crime before calling police.