−18 is not arbitrary and it is not really a food-safety number in the way people assume.
Microbial growth has already stopped well above it — nothing meaningful grows below about −10°C. So −18 is not "the temperature where food becomes safe". It is the temperature at which the chemical and enzymatic reactions that ruin quality — fat oxidation, colour changes, texture loss — become slow enough that food keeps for months rather than weeks. It got standardised internationally, and once it is on every appliance and in every guideline it stays.
Which answers question 3 in a slightly disappointing way: colder does keep improving quality, but with rapidly diminishing returns. Going from −12 to −18 is a big difference in how long something stays good. From −18 to −25 is a small one, and it is smaller than the difference made by how well the thing is wrapped.