Will the Genius Bar help me transfer my files between macs?
As a former Genius, I can say that we were always willing to lend cables to customers for in-store use, but we would not overtly assist with data exercises. Then primary reason is the legality: if Apple gets involved with your data, they become liable for your intellectual property...which can be a legal mess. It's the same reason Apple doesn't offer a guaranteed backup/restore service during a repair. 

So, my advice would be book a Genius Bar appointment, explain your situation to a technician, and go from there. The worst they can say is no. 

P.S. Unless Apple Retail Store policy has changed, you have a 14-day return policy. I had customers buy a cable, use it for a few days, then return it due to "buyers remorse." ;)