The police suspected him within days at the outside, yet there existed no evidence at this point. With all due respect, you vastly underestimate the powers of the police to massage the evidence to fit a chosen hypothesis. Eyewitness evidence is quite malleable, a point that I have made at length in another thread. I find it extremely difficult to see how this investigation could be judged as anything other than inadequate. The incompetence started with respect to the management of the crime scene, but it did not end there.
It was managed adequately enough. Even the eminent Craig Dobbie said so. You tell me a perfectly managed crime scene anywhere in the world, and I'll tell you the sun rises in the west. Perfection in police investigations & crime scene management doesn't exist and will never exist -- that is common sense.
So, Jodie's body was left uncovered in the rain for approximately 8 hours, her body was moved from its original position slightly and her clothing was all bundled together in a heap as opposed to being bagged separately. What difference could this have made? Her body being exposed to the rain and elements was hardly going to wash away vital dna evidence, was it? And her clothing being all bundled together instead of bagged separately was hardly going to contaminate the crime scene to the extent that vital dna evidence would be lost, was it? Let's be realistic here and apply some common sense. The forensic teams deployed to the crime scene and laboratories for analyses were all using state-of-the-art equipment that could detect the tiniest traces of dna, so anything freshly deposited at the crime scene, or anywhere else the police decided to check during the investigation, would have been detected and used in evidence. No incriminating DNA was found anywhere. And the SCCRC retested everything circa 2013/14 and, again, nothing incriminating was found. End of. All of this highlights why discussing the DNA in this case is pointless.
And then, of course, you have the rest of the circumstantial evidence, including the positive eyewitness identifications (who also identified LM in court). And not one of them saw another male similar to Mitchell on that road that day (of course, I look forward to reading the court transcripts of DH & MO). And, I don't care what anyone says -- AB positively id'd LM on the Easthouses end of RDP that day at 1655 (she even told police confidently that "she was as sure as she could be" that it was him, when shown those book of photos; and, of course, she said in court she wasn't sure if it was LM in the dock -- not that it wasn't him, for she was simply being honest, because he'd changed so much between her sighting of him and her court appearance). Overwhelming circumstantial evidence to convict LM, and far too many coincidences for it not to have been him.