I have taken almost the same approach with a soft power down circuit with a overlay for requesting (input) an power down and and overlay for providing the pi status (output). Described in this article.
A1: Yes
A2: Yes it gracefully shutdown the entire OS and applications running on the Pi.
A3: No need for it. See A2.
A4: No need for it. See A2.
A1: Yes
A2: Yes it gracefully shutdown the entire OS and applications running on the Pi.
A3: No need for it. See A2.
A4: No need for it. See A2.
