It could also be that faradays mom Ms. Hawking sent faraday back through time before he even went to the island and that is how he lost his memory.( We all know faraday experimented on him self.) While Faraday was doing his reseach back at oxford when desmond visited him His mom could have simply said go back tell dr.chang this (enter comic con conversation here.) and then go see a large man in jail (hurley) he'll think your a ghost cause he can talk to dead people. He'll know you but you won't know him act like you do, convince him to return to the island and make sure he brings.... (what ever is in the guitar case.
Just another thought you never know. Time will tell.