Hi Noa
It's hard to say how much heat is lost through the head if you don't wear a hood. As you say there are many theories and some people seem to be able to dive without one in pretty cold conditions.
My mate Chas sometimes doesn’t use one in 11-12 degree water (which would freeze my brain in a hurry

)
When I first started freediving, I used a semi dry wetsuit with no attached hood and I got really cold, even though I wore a separate 5mm hood.
The biggest problem was the way the water entered my suit through the neck. This flush of cold water on every ascent and descent negated much of the effect of the wetsuit.
If the water was warm enough, this would be a minor issue but even in 21-degree water, I was very cold after an hour or so.
If you could get a wetsuit with a really effective neck seal that didn’t allow the water to flush through the suit on every dive, then you would probably be Ok.
I think you would have to be careful to make sure that the neck seal wasnt too tight.
Cheers
Ash