The extra complexity spreads out the load. All of the things that often fail in an engine are less likely to when the engine only operates for a limited duty cycle, for extended durations, and for a fixed optimal load. You have the more robust motor system smooth over all of the transients. So while there are more single point failures the system might be overall more reliable.
If simplicity was the only way to make something reliable we would have rickshaws rather than cars.
I also think this is correct. Apart from the fact that any reasonably modern cars will usually go into limp-home modes, or even stop driving, if errors are detected, I seem to remember something about an electric drive motor needing to spin backwards when driving using the combustion engine. That feels wrong as I write it, but I remember it being something fundamental to the planetary "Power Split Device".
Depends what, but mostly yes. If the battery fails then the engine is used permanently. If the engine fails then the battery can be used for its limited range.
