You're missing the point;
Shane Mitchell knew what he had been doing (whether or not he was embarrassed is irrelevant) It may have appeared to come as a shock to him when asked about it on the stand during his brothers trial but let's face it, he must have known there was a chance the police would find out when they looked at his computer and the prosecution would raise it during the trial.
It was Shane Mitchell's decision to choose to keep this evidence from the police; I presume he also knew right from wrong? I'm referring to his omission of evidence btw.
All he was asked to do was corroborate his brothers alibi. He didn't!
How have I missed the point? Shane didn't have experience of giving evidence in a murder trial. He was embarrassed by the porn revelation. He couldn't say that Luke was definitely in the house but he also couldn't say he definitely wasn't. So he may not have been able to support the alibi but he didn't refute the alibi either.
Correct me if I'm wrong but all charges were dropped against Shane and his Mum.
Will you answer a question now? Why do you attach no significance to the rest of the search party all completely changing their statements regarding Luke's behaviour that night?