I started off thinking, oh why doesnt he probe more and ask more difficult questions but there was a great post on here ( yesterday ? ) from someone and sadly I cannot remember who, but it explained in good detail as to how much the Prosecutor can do.
I think Mr M has done a great job of showing just how ridiculous both NM and SHs stories are, and he ( or any Prosecutor ) has to trust that the Jury can see the relevance of his line of questioning and how he is revealing their lies.
I think if he were to ask NM and SH how and why they killed Becky and what they were hoping to achieve, then they are not going to answer him as it would only incriminate them further. So he would end up with cant remember, dont know, type of responses and ultimately it would begin to look as though he were badgering them to no avail.
I think it's a fine line between trying to get NM and SH to reveal as much as possible but knowing that, unless they crack, they are never going to tell the truth about what happened and why they did it.
But,as was said on here earlier, if Mr M can create the situation whereby the Jury do not believe a word of what they are saying, then the only option left ( hopefully ) is for the Jury to find them guilty.