So, here is a thought on the BBT idea that Indy doesn't matter to the plot of the first film.
1. If he doesn't get involved Marion dies when the nazis get the amulet.
2. if he is not there when they open the Ark, then the next group of nazis sent to investigate the missing troops gets it and takes it back with them.
However, if Indy doesn't get involved in this film, and instead just gives his half to Wombat, then the nazis still go back in time and still fight the Romans for Archimedes and still crash and die. So the only downside is that the Dial doesn't get returned to the future.