https://youtu.be/jI2zxBAIepI

But, “speedy” relative to what? Relative to the walls of the room your are inside? What if you are in a falling elevator? Relative to the rotating surface of the earth? To the center of the solar system? “Relative to the portal” is the only answer to that question that makes sense.

How can it not be b? Every situation in the Portal games is already exactly like this, but with the portal fixed to a slab that moves with the rotation of the Earth, whereas in the drawing the portal moves as the sum of earth rotation + the movement of the train.

Treating the question seriously, I think it is easy to think the one person is the right choice, because it just seems like less of a mess to deal with. However, I also think the end result is that you will never feel safe in your home again. You will always second guess if every squeak you hear is yet another person who somehow have made it into your house. This is the path towards slowly descending into madness.