> but then it could be absolutely correct to say “the many hairs on my head”, an uncountable which retains the plural.

That's not an uncountable. It's the count form, explicitly being counted by the count-noun-exclusive determiner many. (The mass form of many is much.)

The entire difference between "the hair on my head" and "the hairs on my head" is that in the second one you're counting the hairs.