Well dang. That does work. I misread/misunderstood the text at
:help 'diff'
which says that the setting is "local to window". In my
(mis-conceived) tests, opening a new vim window (:sp) from a diff'ed
window resulted in a windo that had 'diff' set, leading me to
(mis-)understand "window" as a vim-session not a vim-window. Had I
properly tested, I would have noticed that after a ":sp" on a diffed
window, I could then
:set nodiff
on one of those and it was in fact scoped to a vim-window not a
vim-session.
Reading up at ":help start-vimdiff" it does indeed say that "Diffs
are local to the current tab page" which is exactly what I wanted
and just failed to find/understand.
Thanks for setting me straight!
-tim