What is the current state of the law on the defence of defence in a murder case?

The appellant was represented by competent counsel who ran a skilful defence at trial. He managed to get the denials of complicity of the appellant in the killing before the jury and avoid the cross-examination of the appellant. The appellant sensibly acceded to the way in which his trial counsel managed his case. Now he seeks to run a different line of defence. This is not something that is usually permissible: see Wu v. R., 1934 CanLII 28 (SCC), [1934] S.C.R. 609, 62 C.C.C. 90. However, even if this hurdle could be gotten over, the proffered explanation is nowhere near the strength of the explanation proffered at trial.