Using diodes to drop the input voltage may cause headroom problems when the battery is not charging.
Does the IPS6011 get hot if the battery is charging, but the load is turned off? How about if the load is on by a very light load, like a 1K resistor? If it gets hot due when off or only lightly loaded, the it would suggest that noise spikes from the alternator are causing internal junctions within the part to zener conduct and consume lots of current. You can try adding additional filtering on the input like an inductor and a capacitor. This should limit the noise spikes.
I would also suggest contacting International Recitifer. They are pretty good at answering apps questions with regard to their devices. For example, the datasheet lists two values for VCC max, 36V and then 28V continuous. Under what conditions do these two apply?