i am very sorry for your loss.
You said that the blood work up came to be normal but still she was feeling short of breath.
I dont know whether any ECG or Chest Xray was done as you havent mentioned.
It would have been easier to diagnose as pneumonia
if an xray was done and if it was done and was normal then probably what she had was a cardiac disease.
The pneumonia that they mentioned after her death could have been due to the ventilator which we called as VAP