When Rakoczy and his two goons are chasing Gwendolyn and Gideon, they are shooting them multiple times with single-shot pistols without stopping to reload.
If Gwen goes back in time (as she does at the end) to save Gideon, and then the Count doesn't get the immortal serum, then, when they go back to the future, no one would remember Mr. Whitman, because he never would've existed as a teacher because for him to be Mr. Whitman, he'd have to had the immortal serum, which he doesn't.