We had cake dummies in the bakery where I worked, and they last a long time done in edible materials. One of them was covered in fondant with royal icing details and it just started fading after six months of sitting. It wasn't directly in the window, kinda off to the side of it. It did get a lot of sunlight and was done in shades of purple which fade easily. I think if you are careful with them, keeping them out of the sun, they will last a long time. The fondant gets rock hard but doesn't change in appearance.
As for the perma-ice and spackle damaging the tips etc. It's fine as long as you wash everything right away. Don't give it a chance to dry hard.