I think it's all very well, one thing, that LM, a 14yr old may have, in the heat of the moment become so enraged he killed his girlfriend. Being able to plan the perfect cover up in the aftermath is something entirely different. "The police didn't bank on Luke being Luke though" - there is however no way that LM could have banked on the police being anything. Of the finer details in everything that would have been sought to confirm all that was said.
What did he need - An alibi, evident at this point that only his mother would perhaps suffice. No evident collaboration from SM. From the moment it was evident however that his mother would not suffice, SM's story changed.
Does it make sense, even after the change - simply no. That he had come downstairs, Luke was mashing tatties, had a conversation around the dinner, went back upstairs, then came down and collected his dinner, took it upstairs, ate it and left shortly after 5.30pm - CM did not arrive home until 5.15pm at the earliest, the dinner was not ready. Where is the time?
We know by Luke's account he could not have left any later really that 5.30pm. He claimed to have made the call to the Jones landline, whilst sitting on the wall at the entrance of the estate. This call was at 5.32pm.
Both brothers, irrespective of change in statements made it clear that neither saw the other around these times of leaving.
How could this possibly be? - SM could not have left earlier than LM and SM could not have failed to see Luke walking or sitting on the wall. There is only one entrance to Newbattle Abbey crescent. It is a straight road to the entrance from the house in which they stayed.