i don't have much to add to your thread, except
use what ever focal length feels right to you.
i shot for a long while for a newspaper head shots
environmental portraits, pretty much everything you can think of ..
and i just used what i had handy. sometimes it was a 28, sometimes it was a 50
sometimes it was a 100 ( even a 135 )... whatever worked ... there was no steadfast rule ..
just general "tips" for positioning good v. bad side general "stuff" i learned
as an apprentice to a portrait photographer a few years earlier.
rules of composition are different now than they were 40 or 50 years ago
for headshots at least, you can cut heads and necks and chins off and no one will say much ... so do what
you like, and if you are doing it for a client or $$ or whatever ... make sure
they like it too
good luck !