Good questions.
You only get one set of cake feet. They will degrade over time. You'll need to use them sparingly and wisely. It will be a thick dense cake, about 10 layers. They will last you about 8 walking years. So not 8 years in total, could be more. If you only walk say half of your year, you'd double you're feet lifespan
You can okay the trombone and have full control of the slides but you wouldn't be able to feel anything with them, for example the feel of another human or petting an animal.