Welcome to the Cake Decorators Q&A
Can unused ganache be stored in the freezer to lengthen its life? Same question for a ganached decorated cake and for leftover cut cake. If I'm not mistaken the tutorial only dealt with storing the ganache in the fridge.
Ganache can be frozen for several months. It has to be be well wrapped in freezer bags and placed in an airtight container and stored well away from strong smelling foods. Cake can be frozen for 3 months. It should be wrapped in parchment paper and several layers of foil, popped into a freezer bag or cling film and boxed up to prevent freezer burn. Bring both to room temperature still in wrapping.
The ganache will have to be gently warmed up on low temp for between 10 - 30 seconds spurts to bring it back to working consistency. Chocolate burns quickly, to prevent this stir the ganache slowly after each warming to distribute the heat. Don't over heat it or stir over rigorously as this will release oils from the chocolate and make it grainy.
Ganache is widely discussed on the site, there isn't a lot about it that hasn't been covered by members.
If you search in the above search box ' ganache shelf life' 'ganache' 'freezing ganache' ' oily ganache' or simililar key words you'll see many answers which will useful in the future.
Hope this helps.