And why would the switch for having a cold room (in which your fridge
should need to work less hard) be the same one for freezing stuff
quickly (for which it should work harder)?

The fast freeze switch makes the compressor come on more, which makes the
freezer part colder. If you install in a cold location, the thermostat,
which is in the fridge part, may never come on, so it needs a bit of a boost
to get it to work. You'll probably find your fridge starts freezing, though.
This type of fridge freezer is not really suitable for installation in
anything other than a heated habitable room.

Christian.