The post it notes are the most unimportant part of the prosecution's case.
There are multiple medical experts who agree that these babies were assaulted or murdered. There are several eyewitnesses and a metric tonne of circumstantial evidence that make Letby the only suspect. I don't see anything that can topple the prosecution, unless the defense can pull out medical experts who can credibly challenge the other expert's views.
There are never smoking guns in health care serial killer cases, and actually many murder convictions generally rely on circumstantial evidence. Real life evidence is rarely as neat and clear cut as murder documentaries make them out to be.
There is actually a lot more evidence than average for a case of this nature.
If she's found not guilty on all 22 charges (and she's certainly not helping herself right now), she will definitely not return as a nurse, there's indisputable evidence that she has broken confidentiality rules loads of times, plus texting on shift, and if found not guilty, then there will be questions about her competence.
Plus, this trial only covers June 2015- June 2016. She was working as a nurse and student nurse from June 2010, and Operation Hummingbird is expected to continue for at least the next three years, investigating the earlier parts of her career. We know that in Liverpool Women's hospital she had two dead babies, one after the other. If she's found not guilty for everything, there's every chance she could be immediately arrested for other charges.