Precisely, there is plenty of Mirrors to move around and the doors to the Reactor technically exist per those rules. So, players can figure out a way to get a Mirror into the Reactor, should they wish to access it from the Mirror World. I like your interpretation of what exists in spaces without Mirrors, it feels like a wall of "void flame" that slowly consumes anything caught in it. But yes, we can look to detail more mechanics of the Mirror World boundaries. This all the more showcases how the module would better serve as a small zine to cover all bases.
This is also the implied reason why the Mirror Entity is asking players to ship out a shipment of Mirrors, as it is using said Mirrors to expand their domain outside of the Mining facility.