I'm not a KC but I suspect what Ben Myers has been doing so far, is to try to project the human side of Letby. Until now, to the jury she has been a 2D character, so he's trying to fill that out. In doing so, by making her more real, he's trying to create doubt that she could have committed such a heinous crime. She must be convincing in saying she's not guilty as he asked her in the stand if she had committed these crimes. It is NOT the Defence's job to prove their side, the burden of proof lies with the CPS. I know you know, but the jury will be told they have to be completely convinced she did it, not pretty certain.
I would imagine he'll have a retinue of expert witnesses lined up to cast further doubt, medical and maybe technical, i.e. door access data etc.