I really think you're over-complicating what is really a non-issue.
Science would have a very difficult time creating a general equation for the loss of heat from a human body.
But that's not what the THI is supposed to do. The "feels like" association is a simplification that got boiled down into a cute catchy phrase, much like "defund the police" and other crude slogans meant to convey an idea which cries out for more explanation.
For every cubic centimeter of water evaporated the surface temperature is cooled one degree centigrade*. The less water evaporated the less cooling. High temperatures cue the body to begin cooling itself through perspiration. Air movement aids in evaporation. But the higher the humidity, the less moisture is absorbed by the atmosphere. At a certain level of humidity, even a stiff wind will not evaporate moisture from a surface. The THI simply (simplistically) gives the average person listening to a short weather report a quick way to gauge how comfortable it will be doing activities outdoors.
I can't believe I'm actually defending a dumbed-down "measurement" used by weather forecasters, whose reports are already significantly dumbed-down for popular consumption. You raise some valid points, and thank you for bringing this to the attention of this forum, but eventually it's like using a baseball bat to dispatch a cockroach. It's overkill. It's just a guide for people who might not otherwise grasp the relation between temperature, humidity, and personal comfort.
As a former science teacher, this is a bad thing.
No, it's not "bad". Nor is it "good". It's not a something to which such values can be applied. Inaccurate, arbitrary, unscientific it may be but it isn't bad. People aren't harmed by it and it's intended to be helpful.
EDIT: *This is the definition of one degree centigrade which I learned in the 8th grade. If it's incorrect, there is still a way to measure cooling by surface evaporation as that is a measurable phenomenon.