In patients with acute pancreatitis about 25% do have urinary tract infections which predate the pancreatic problem or is a concomitant feature.
Not necessarily, acute pancreatitis may start insidiously and not necessarily present as the explosive condition for which it is noted.
Antibiotics play no part in the treatment of acute pancreatitis but may well be used to treat associated infections such as UTI as was the case with your father.
I cannot comment on why the antibiotics didn't work except to say that your farther's symptoms were mainly due to his pancreatitis and not his water infections.
The early stages of pancreatitis canbe difficult to diagnose,but even so, pancreatic enzyme levels which if elevated should make acute pancreatitis an early and obvious contender for the diagnosis.
Computerised tomography would also help in the diagnosis.
Anything can happen in any way and sudden collapse would either suggest a blood clot in the lung or a heart attack either of which could have been picked up at post mortem.
Sudden collapse as you have described it is unusual in sepsis, but as I have said....anything can happen in the field of Medicine and Surgery.
You are clearly concerned and I am sorry for your loss.