100 is the imprecise average body temperature of the developer

That's a myth. It's no more true than the myth that it was the body temperature of horses, or that the scale was designed to reflect how humans experience the weather. (It happens to reflect how humans experience the weather, but this was an incidental characteristic and not the purpose for which the scale was designed.)

The Fahrenheit scale starts to make sense when you realize he was a geometrist. It turns out that a base-10 system of angular measurement objectively sucks ass, so the developer wasn't particularly interested geometrically irrelevant numbers like "100", but in geometrically interesting numbers like "180". He put 180 degrees between the freezing and boiling points of water. (212F - 32F = 180F)

After settling on the "width" of his degree, he measured down to a repeatable origin point, which happened to be 32 of his degrees below the freezing point of water. He wanted a dial thermometer to point straight down in ice water, straight up in boiling water, and to use the same angular degrees as a protractor.

The calibration point he chose wasn't the "freezing point" of the "random brine mixture". The brine was water, ice, and ammonium chloride, which together form a frigorific mixture due to the phase change of the water. As the mixture is cooled, it resists getting colder than 0F due to the phase change of the water to ice. As it is warmed, it resists getting warmer than 0F due to the phase change of ice to water. (Obviously, it can't maintain this relationship indefinitely. But so long as there is ice and liquid brine, the brine will maintain this temperature.) This makes it repeatable, in labs around the world.

And it wasn't a "random" brine mixture: it was the coldest and most stable frigorific mixture known to the scientific community.

This criticism of Fahrenheit is borne of simple ignorance: people don't understand how or why it was developed, and assume he was an idiot. He wasn't. He had very good reasons for his choices.