I don't believe in the "fading" memory theory. How Al, Ziggy and Sam can remember the previous history after it has been changed is because I see them as anomalies in a new/alternate timeline. In other words, they're simply left over products of the original timeline (or universe) of which they came from (which would be before Sam made his first leap in May of 1995). Because of the brainwave connections between the three of them, this allows Al and Ziggy to be anomalies as well, and prevents paradoxes from occurring each time Sam completes a leap (i.g. once Sam saves someone in a leap, there now becomes no reason for him to have leapt there, and each leap turns into his first).
But this leads deeper into my theory that there isn't one single, rewriteable timeline like the show suggests. I believe it's more of a parallel universe thing in which each change Sam makes branches into an alternate history. I don't believe even our main characters necessarily recognize that this is the case, and are under the impression that there is a single timeline. For example, the Sam we know would vaguely remember saving Tom in "The Leap Home Part II── Vietnam," but wouldn't have memories of growing up with him, because "our" version of Sam never did. This is where the anomaly comes into play. After Sam's leap, the new history after April 8, 1970, would have a slightly different version of Sam growing up with Tom around.
So, even post-"Mirror Image" Al, who is married to Beth with four daughters, would still have a memory of what his life was before Sam made the change. The only problem is how Al doesn't seem to remember that he was executed in "A Leap For Lisa." While Al didn't seem to be aware, Sam was aware. Perhaps Al had no memories of being dead because he actually was dead. Al would have the memory of having a 99 percent chance of becoming executed, but that would be it. (I probably have to watch "A Leap For Lisa" again because I can't remember the details well enough.)