So, technically, if 50+ mindless undead rushed through the threatened space of a nonliving good guy towards the living players (the only way into the room), the nonliving good guy could get 50+ attacks that round.
Yes.
This may break your idea of verisimilitude, but I think so would having 50 mindless creatures all pass through the same five foot space within 6 seconds and not fall all over each other is also pretty hard to believe.
(And technically you move faster if you go diagonal than if you go straight forwards backwards, or sideways. There are many things we need to accept in order to keep the rules manageable.)