are they edible after all those months?

maybe done in fruitcake but generally no and the caker would not want it eaten--they usually are made of foam

No, the competition pieces--unless specially carved--are made of styrofoam dummies.

When I did a carved cake for competition, I used a commercial cake mix but didn't use dairy in it.  It held up for a long time, but eventually succumbed to mold.