I don't think they can ask for it at a later date, it would have already been presented as an alternate charge during the first address to the court. By claiming she was overworked or unwell she would possibly be saying she did her best and perhaps she feels her best wasn't good enough rather than admitting to being at fault.When you say ‘the deaths are not her fault’ technically they still would be her fault I think? she is responsible for taking care of them so she should have still thought duck I am making too many mistakes and then took the responsibility to go the sick? So if they were going to go down that route would they not be asking for a manslaughter charge or can they ask for that at a later date?
I'm just trying to think of the arguments her legal team could make because 'I didn't do anything wrong' alone, doesn't seem like a particularly strong defence in the face of what appear to be events that have occurred with such frequency and in such unusual ways that it can't be explained by accident or coincidence. On a the basis of 'on balance of probability', it's not looking good for her at this stage.