My guess is there was no key.
Those aliens just invaded that day, sun high, that place.
The effect thought to overload the space continuum was bullshit, nothing happened, aliens just arrived there and then.
First time, young bunmei was alone, he wasn't able to do anything against the aliens.
First loop, adult Bunmei snatches young Bunmei's power, does a full power release that kills all aliens, closes the passage and he dies in the process. End of the Alien timeline, end of the loop.
What do you think?