I've just had that same idea. His bumbling going up stairs, falling over on a bicycle, and rambling speech patterns......I wonder if the White Hats installed him and he's just playing the part of "Uncle Joe" to perfection. Could Joe be under the control of White Hats? Think about the implications to those who surround him. Would not one of his long friends know he's been switched out and spill the beans? All someone would have to do is ask the new Joe a question about his past and watch his reaction. One of his long ago acquaitences surely would have a story the only himself and Joe would know about...."Hey Joe, remember the time we went on vacation with our spouses and you pissed off the end of the pier and a fish tried to nibble on your manbits"? Some sort of recollection that is so ingrained in everyones memories that it could not be a lie nor could Uncle Joe not know the story, unless he's a replacement.