I have never used ganache to fill and frost a cake. Does it crust? Is it easily spreadable?
It does get solid, but I'm not sure I'd use the word crust. When you make it to ice a cake with, it will be the consistency of peanut butter, so yes it will spread.
Can the ganache be used under fondant?
ganache works great under fondant. I like to make it the day before, let it set up over night, then whip it into a nice creamy frosting. it doesn't crust but it sets up nicely for fondant!