I believe with all my heart that GA did see KC and Caylee leave on the 16th. At the time he talked to the FBI and told them that he had no reason to lie. He was sharing everything he could and not holding back anything about KC. He even admitted he thinks there was a good chance it could have been an accident. He was very sure it was around the time he watched his cooking show and just seemed to be very truthful. At this point they did not know Caylee was 100% dead and believe he was trying to share everything he knew to find out the truth.
He had every reason to lie: he'd already smelled the decomp odor coming from the car on June 24, when he and Casey got into it next to the car about the gas cans. He was already in "cover for Casey' mode," and he knew CIndy expected him to do this. (Heck, she'd probably threatened him.)
ETA: Remember that by the time George talked to the FBI, he'd had plenty of time to get his stry straight with Cindy...and you can BET she told him what he'd better say.