Celsius is also kinda arbitrary, but at least it sets its 0 and 100 to very fundamental, observable temperatures, namely the points where the state of matter of water changes. There are more constraints to it of course, in particular atmospheric pressure, and the modern definition of Celsius is actually purely based on Kelvin (which in turn uses the Boltzmann constant), but as long as you're not high up in the Andes, everybody can observe a pretty good approximation of it.
It... isn't. That would change wildly depending on which sea/ocean you get your saltwater from (more salt = colder freezing point).
It really is defined relative to a very specific brine mixture (in the most scientifically generous origin story - some say he literally just measured the coldest winter day he could). Well except it isn't anyway, because like all US units nowadays it's defined against metric units (namely the Kelvin, just like 0°C is actually defined to be 273.15 K).