The ending of The Philadelphia Experiment II is of course impossible; as we saw last time there is no future to which Dave Herdeg can return. However, in that last scene there is a detail that reminds us of another problem, a very significant problem, with this film. Jess is in the stands at the little league game.
To refresh your memory, Jess is the girl who leads the resistance group that rescues Dave from the military police, takes him into their custody to deliver to Longstreet, and leads the raid which puts him in the time vortex to travel to 1943. This last she does with full knowledge that if he is successful, the world she knows will never have existed. She is probably the closest thing to a romantic interest the hero has in this film, and the hero is going to erase her entire universe.
Yet there she is, at the baseball game.
This is doubly odd because in the altered history she indicates that she is sterile, a casualty of the bomb. We do not know her age, but she is clearly considerably younger than fifty, so she was born quite some time after the 1943 attack. At a Little League game, we assume she is mother of one of the players. That means her sterility in the other history is a genetic defect from the radiation; it also underscores our genetic problem, a problem which we ought to have noticed in connection with Mailer. We have an original history in which Americans, mostly men, died in significant numbers fighting a war in Europe and the Pacific. We have an alternate history in which those men probably did not die, but millions of Americans died when bombs were dropped on Washington, D.C. If Jess is thirty-five years old, she was born in 1958; that is fifteen years after the bomb dropped, and it is highly unlikely that her parents would have met in both histories, never mind had the same child.
Similarly, the Mahler who is a German war hero blamed for the loss of his superweapon will have a very different life from the Mahler who is a German scientist grabbed by the American government to do research in America. That this man would have had the same child in both iterations of time, even if we allow that he may have been married already in 1943 and that his son was born late in his life, is incredibly improbable. We know that he was childless at the pivotal moment in his life; for everything else in his life to have changed except the son he has not yet fathered would only be possible if Mailer himself has already been conceived and his father does not know it, which makes our future scientist forty-nine years old--possible, but it does not seem so. It would also mean that the death of Mahler would not be the undoing of Mailer.
Add to this that Allison married Dave in the original history, but not in the alternate history, and we again have the problem that relationships have changed--not only has Benjamin not been born, it is likely that an entire generation has changed noticeably, as Allison marries someone else who does not marry whom they would have, who marries someone else, and the world is very different. Count Jess in that change as well--if indeed she is a Little League Mom, she has changed the list of who is born in this world. You cannot have a parallel universe in which some things are seriously different but the same people are born at the same times and places anyway.
So although it would be nice to imagine Dave finding a new relationship with the girl he met under very different circumstances only he remembers, on some level it merely adds insult to injury in a film that has been impossible repeatedly throughout.