Does anyone ever begin to decorate a cake while it's still frozen? Crumb coat, fill and ice with buttercream?

Yes, the bakery I used to work in we did it all the time. We just used either the pointy part of the nail or scissors to poke holes in the cake to help with the expansion as it thawed. If you don't give the cake somewhere to go it could cause issues (and even if you do it still does sometimes.

But it doesn't take that long for a cake to thaw to a cold cake. Usually in about 15-20 min I can have a cake that is almost compeltely thawed.

I've crumb coated and filled while frozen. Sure makes it easier. But I only put it back in the refrigerator--not the freezer-- while I add layers of butter cream until I get it where it needs to be. The cake thaws but it's still cold. With each additional layer I'm able to smooth it so I've never had a problem with cracking or deflating butter cream--even after it's softened.