Ok, it is just a replacement/modification of s6-shutdownd
yes , this API do not come from me and so 66 doesn't take ressources. It was renamed to 66-shutdownd to avoid confusion with the original one and to be consistent with the other 66 tools name.
@ 1
It handle correctly the shutdown process which create on the fly the stage4 and handle the 66-umountall program(s6-umountall). As it a independent program which do not depends on any tree it can be survive until the last second of the shutdown procedure.
@ 2
You will loose features like the ability to program a shutdown at specific time. Also, you can reboot or shutdown using the 66-hpr program but you need to use the -f option
@ 3
well if you use 66-hpr -f an immediate shutdown/reboot is executed but it only make a sync. it do not stop service, umount disk, ....