... weighs one gram ... An amount of hydrogen weighing the same amount has exactly one mole of atoms in it.
Not only was this never true - the sentence would have to have say "An amount of carbon-12 atoms weighing 12 times this amount has exactly 1 mole atoms in it" (far less elegant) -- but not even this is true any longer after the fuckup in redefining the mole in 2019, after which all these relations between amount of substance and mass are only approximate.
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.
Nope, this redefinition isn't necessary, it is a choice SI made. Nothing would have broken by keeping an exact relationship between amount of substance and mass, it would just have retained the interpretation of Avogadro's constant from before 2019 (experimentally determined vs a defined constant).