The 12V battery runs most of the normal accessories that it would in a non-hybrid such as radio, lights, horn, blower fans, coils for the ICE spark plugs, engine cooling fan, all the electronic control systems like emissions and ABS, etc. I guess a sign it would be failing would be these systems not working properly. You could always test the battery with a load tester(?) There is a device that steps down the 245V DC to 12V DC to charge the regular battery.

I believe there is no HV-to-DC converter as the HV battery is 245V DC. That voltage goes to inverters located above the transaxle where it is converted into AC and higher voltage for the traction motors. I also believe the AC compressor is run directly by the 245V DC - at least it has some big orange cables going to it which means don't touch.