For a 96 Saturn SL1, is the engine all aluminum or is it an aluminum
head with a cast iron block?

Has anyone on the list had a head gasket fail on them at high mileages
(~175,000) even though the car was never overheated, coolant/oil was
kept changed?

I can't figure out why the head gasket failed on this car (oil gets
into the coolant... it runs just fine) but I am wondering if the cyl
head is aluminum and the block cast iron, that perhaps the two
different metals somehow caused the head gasket to fail after all the
heating up and cooling down with many miles of normal use.

I've never heard of a head gasket failing on a car unless it was
overheated.