I am so sorry for your loss. Actually, you can get septic with E. coli. She may have had the E.coli that was in the urinary tract infection get into the blood stream or she could have had a problem with her bowel such as a tear and the E. coli (which is normal in the intestines) get into the blood stream. They would then give antibiotics and try to get rid of the E. coli in the blood. E. coli isn't always food borne. So, it depends on the situation. I hope this helps you resolve your question. I don't think they were withholding information rather it sounds like it was a problem she already had and it got worse. The death certificates often say what the main thing that caused her to pass at that time, even if it isn't the whole picture.
did you notice if others in the family got sick around the same time she was?
you know it's possible that they don't really know the exact causes of death.
if they found e.coli infection that could have possibly been the primary infection or rather secondary to a viral infection that challenged her immune system.