I took out my cakes that were frozen, cut them and put BC in the middle to combine the layers. I left it out to go back to room temp. how long do I do this for? Then after how long is it ok after I have the Fondant on it for it to stay outside in room temp in the kitchen?

I can't address the fondant part of your process, but you can start icing with buttercream while the cake is still frozen and it just defrosts as you ice. And most people finish decorating the night before a cake is due, so overnight is certainly fine.