I would like to request a vimscript function or api function that should negate the effect of the --headless command line switch. Lets call it nvim_launch_builtin_tui() for now. I imagine something like this:

if My_condition()
  call nvim_launch_builtin_tui()
  " now it looks as if we did start nvim without --headless
else
  " this text will go directly to stdout
  echo "running headless"
endif'

and then one would do nvim --headless -S above-file.vim. Depending on the user function My_condition one would effectively end up in a "normal" TUI nvim instance or in a headless nvim.

If I can express the logic of the My_condition() vimscript function in the my_condition shell command/function I can currently do something similar with this shell script:

if my_condition; then
  # this now *is* a nvim without --headless
  nvim
else
  # this text will go directly to stdout
  nvim --headless --cmd 'echo "running headless"'
fi