I disagree with you entirely that hearing from medical experts with opinions that counter/ challenge Dr Dewi’s testimony wouldn’t have helped her defence. The barrister seemed to think that he could poke holes in the expert testimony without a need to put their own expert on the stand . I think that was a grave miscalculation
Not having been on her defence team, neither of us know what factors went into the decision not to put their own expert forward. She had an experienced, well-respected barrister so I'm inclined to believe that her team had good reasons for making the decisions they made to give her the best chance of avoiding conviction.
However, the notion that it might have been a miscalculation on their part is somewhat supported by the fact the defence attempted to bring in expert evidence on appeal (although I don't believe they never even considered bringing in Dr Lee for the original trial - I think it's more likely they went with one strategy that failed and then, when they had nothing to lose, tried to get a second chance to try out a different strategy on appeal).
I suppose my point is not that hearing Dr Lee's (or another defence witness's) evidence at the original trial wouldn't have helped Letby much, but more that this is what her defence team decided, based on the detailed inside knowledge they had of the case (that we will never know).
The fact they could have called witnesses but didn't will have been based on their inside knowledge of the case and risks to Letby's defence case.
We will never know the reasons they chose this strategy, because that may well have been the point of this strategy - to prevent other facts from being admitted into evidence.