Letby wasn’t in a position to challenge the expert medical testimony! Hence her agreeing that there must have been a poisoning with insulin - disaster.
Yes, I agree - unless her defence was that they had got the wrong nurse (or unless she was guilty), it makes zero sense to concede that someone had poisoned the babies but that it wasn't her.
It has been well reported that the jury only heard from prosecution experts- as you yourself acknowledge in your post.
Yes, the defence didn't offer an expert witness, but that was the defence team's choice to make. It doesn't imply that the jury haven't heard both sides of the case- they have heard what the defence chose to put forward.
The defence had ample time and ability to put forward a witness (as explained in the appeal judgment) to rebut the prosecution's arguments, but they chose not to (presumably because of the risk they would be running by doing so). They made a strategic decision and it didn't work out.
They argued there was no case to answer on the air embolism issue, the judge told them that there was a case to answer and that the jury would hear the air embolism evidence, and then the defence decided not to really answer the case at trial anyway. Then, on appeal, they belatedly decided that they wanted to rebut the prosecution's air embolism evidence with an expert witness, and were understandably told that it's too late now. They made a strategic decision not to call a witness, but they could have called one at trial if they chose to.
Realistically, though, even if they had called an expert witness, it wouldn't have actually ruled out air embolism, and the prosecution would have been able to use them to support their case too, so it likely wouldn't have helped with much anyway.