I wouldn't use chocolate ganache without consulting the customer first-- esp with a spice cake. I havent made wite chocolate ganache, but i do know the ratio of cream to choc is different.
For success with making ganache, make sure you heat your cream but pour it over your chocolate that is in a bowl (i use glass)-- do not add your chips to the pot of hot cream-- you will get lumps. So, pour the cream over the chocolate, let it sit a few minutes, then use a whisk to combine. Also, let it cool gradually... Cooling too quickly can cause an off texture as well.
Yes, the ganache gets 'hard'.. But not too hard. You could use buttercream i guess and keep it well chilled. You'd want a 'stiff' buttercream and a good crusting recipe for the outside. Support is key for stacking as well.
As for carving, i use doctored mixes and This is a great tutorial http://tinyurl.com/Flikrtut
Eta: the ganache should not be pourable when you ice the cake if that's what ou choose to use-- it should be chilled/cooled to an icing consistancy. I have read that planet cake wont do topsys in anything but a dense mud cake with chocolate ganache. I will do them in any flavor, but will only cover in ganache + fondant or ganache + buttercream. (& i strongly prefer the fondant route)