Welcome to Applecare. The "genius" section of the returns area in the store consists of simple retail workers that are anything but. It was serving coffee at Starbucks, or returning Apple products at the retail store..... whichever job called back first. When Applecare tech gets ahold of it and gets it apart they SHOULD be able to see the cause. It'll be a very hot spot on the motherboard and components will appear blue or darker in color (discolored basically) that caused the issue. Apple only trains the retail employee's enough to know deceptive practices so they are literally trained to scrutinize you, beyond that their technical ability is..... oh quite limited.
Not really. This may be the case for some Apple retail stores, but a lot of geniuses really are good computer technicians and enjoy helping people and cutting them a break when able to.