What's your evidence that she was able to do it though? Nobody saw her or anybody else with a syringe (or filling a syringe multiple times), the doctors on the video had to look up how much it would take (no evidence of her looking for that) so, as you say, it's a bit of a guess how much although 20-40ml is their ballpark figure. They're pretty clear that you'd be able to see that volume of air on an echocardiogram (which at least 2 of the babies had at the time) and it would take a fair amount of time to put that amount of air down the tubes - how much time we don't know because, also as you say, it's not been tested and also that it would cause a very quick collapse, after which resuscitiation would not be possible.
To accuse someone of murder by this particular method, you'd think the prosecution would have to be a bit clearer on exactly how it would work and have some positive evidence that it actually occurred. Just being there (when "there" is the place you work) shouldn't be enough.
Dr Evans is insistent that air embolism is the cause of harm because of the rashes/no ability to resuscitate successfully/baby died. He claims to have ruled out all other possible causes and identified particular time periods when harm was caused, which is where his conclusions conflict with what the other experts and the original pathologists thought. Dr Evans saw the situations as babies who were otherwise fine suddenly collapsing for no apparent reason. He thought that the doctors at the hospital were good at resuscitation, leading him to conclude that some deliberate harm must have been done. This is the argument that was put to the jury so it's not surprising that LL was found guilty.
The other experts, reviewing the same notes and evidence, concluded that there were signs that the babies were not doing well and that care was substandard. Evidence that wasn't part of the trial, like the RCPCH review, the consultants not doing as many ward rounds as they should have, the staff/skill shortage, the baby who died after not being resuscitated properly the year before etc. would indicate to me that the expert panel are more likely to correct in their assessment of the situation.
At this point there are many people who have come forward to say there are serious questions to be asked about the medical evidence and none AFAIK that are backing up Dr Evans' position. There were other prosecution experts but he was the main one and also the investigator for the police. Dr Bohin reviewed his conclusions but didn't do an independent investigation and the others took very little part in the trial. This means that a heck of a lot of the prosecution evidence rests on the work of one person.
Turning this around for a second. If you were LL and you hadn't done anything wrong, how would you defend yourself against the evidence put against you?