Last time we stated that this was the wrong Dave Herdeg. It is impossible for any version of Dave Herdeg to be here, in the altered version of 1993, but if it were possible it would be because Dave fell off the boat in 1943 and emerged from the vortex in 1993 because of American Mailer's experiment. The film does not see it that way, though, and it matters to the plot that this version of Dave Herdeg remembers arriving in 1988, marrying Allison, and raising their son Benjamin, and that he, alone, remembers the world in which no stealth bomber reached Weisbaden and America won the war. Thus we need to understand how that could be.
The excuse given in the film is that because of his genes Dave is the only person who has ever survived time travel. It is, as we saw, twice nonsense--once because until the moment he arrives in 1993 no one in this version of history knows that he ever traveled through time, and again because the bomber pilot must have landed his plane safely in Weisbaden, and thus survived long enough to do that much at least. Thus we can discount the "unique genome" explanation. There is also Jimmy in the first film, although I suppose since he was sucked back through the vortex and returned to the past perhaps he does not count.
It might be argued that Dave and the pilot, not in any way related as far as we know, shared a gene or several genes such that they were the only two people who could travel thus. However, it would be passing odd if the unknown pilot had this trait and Benjamin did not. It is not impossible, but it strains credulity.
Perhaps, though, it is because Dave is otherwise missing from this world. That is, in the original history Dave leapt from 1943 to 1984. (Jimmy did as well, and then returned to 1943 with the ship, creating a second history, but Dave is unaware of the change.) In this new history, created by Mailer's mistake, Dave never arrived in 1984, and therefore there is no version of him in 1993 in this world. Thus he is replaced by his self from the erased history.
While that sounds nice in theory, it faces a significant problem: there are millions of people alive in the other world who are not alive in this world. Remember, Washington was hit with nuclear weapons; those people died. Jess claims to be sterile, victim of the bomb; how many were never born because their parents were sterile or dead? Oh, but never born is different from no longer alive. Dave was born in 1917 and still alive in 1993, thanks to a forty-one year leap. He, though, would not be the only person still alive from 1943--it has only been fifty years, and a substantial portion of the population would have been fifty-one or older in the American 1993. Many of them would have died in this version of history--but then, would that not have included Dave Herdeg? Other than his temporal hop, there is not much to distinguish him from millions of others. Indeed, there is no reason for Dave to have appeared in this version of 1993 that does not apply to someone else somewhere, and probably to so many others that Dave would be lost in the crowd.
So it does not make sense under its own theory nor under any other theory. Dave Herdeg cannot be here.