I had 2 "roses" fall off a 10" x 6" round cake. I washed my hands, and using a butter knife and my hands, gently pushed them back into place. It only happened on this one cake, but now I pay more attention to the details.
Some of it will come with practice and you will find out what works best. The consistency of your frosting, the "stickiness" of the cake surface or crumb coat surface, the technique of piping just close enough and with enough pressure to get the rose effect without problems. Just do a few more cakes and you'll have it down.
Re: Defrosting the cake.
When you put it in the freezer, ideally it should have been placed in a box just slightly larger than the cake and cake board. Personally, I go for overkill, and then wrap the box in plastic wrap or aluminum foil. When you remove the cake from the freezer, LEAVE it in the box, and LEAVE on the plastic wrap/foil. As the cake is defrosting (1-2 hours at room temperature), the condensation will form on the outer layers of plastic wrap/foil and box, NOT on the cake inside.
If you haven't pre-wrapped, you can still do that immediately when you remove the cake from the freezer and it will have the same effect.
Prepare some matching buttercream in a piping bag with the appropriate tip, have it at room temperature, and if something does fall off, you can do some repairs and nobody will ever know.