Let me enunciate a couple of universal principles then apply them to two issues raised in recent comments. One, the testimony of a witness who is cross-examined is more probative than the statement of a possible witness who is not. Two, when interviewing witnesses, the police should treat witnesses without regard for whether they are the prosecution's witnesses or the defense's witnesses.
Applying principle one to the alleged sighting of stocky man, the importance of this incident is what it indicates about the police, more than what it might indicate about the identity killer. There is evidence that tunnel vision set in within the first hours of this investigation. "The number of identified wrongful convictions and false confessions is mounting, and in almost every wrongful conviction, the problem of tunnel vision is present (Findley, 2012; Findley & Scott, 2006; Martin, 2002). According to Martin (2002), tunnel vision distortion is particularly damaging in the investigative process. Investigator’s misconduct becomes prevalent in note and record-keeping, witness interviews, the interrogation of suspects, and the conduct of searches."
https://journals.sagepub.com/doi/10.1177/21582440221095022. The examples of tunnel vision in this case are not limited to the apparent failure to investigate these witnesses in a timely and thorough manner.
When SM and CM were charged with perverting the course of justice, the only evidence that they had lied was the sincere but synthesized (using Professor Simon's terminology) statement of AB. Prior to that FLO Lindsay and other investigators kept saying that they could not accept SM's answers and put words into his mouth (p. 305 and p. 318 in
Innocents Betrayed). BTW, if someone does not trust these passages, let him or her quote his testimony directly. SM's revised account (that he had stopped to help fix a car) was buttressed by his friend's account. In contrast none of the prosecution's witnesses were charged with perverting the course of justice, despite several who changed their account and at least one who did not come forward until long after he should have. Given this disparity in how the witnesses were treated, no wonder that SM prefers to keep a low profile.
EDT
Jenna Macfarlane reported, "He [Shane Mitchell] was then asked by defence QC Donald Findlay if there was any point in the evening where he could say he knew for a fact that Luke was not in the house.
Shane responded: 'I can't say that.'"
Based on this passage, I conclude that Shane failed to corroborate Luke's alibi, not that he contradicted it. As my previous comments explain in more detail, the way that the police questioned him is at least part of the reason for Shane's lack of recall.