If you kill ffmpeg process it will exit with code 255 (easily reproducible in the terminal)

Both kill with a job specifier and terminal signals send to the entire process group, so both the shell and sleep.

rkill command from pslist package sends given signal (or SIGTERM by default) to specified process and all its descendants:

Why then sending the SIGINT manually to the shell doesn't kill the child, e.g. 'kill -2 <shell-pid>' doesn't do anything to a child process while Ctrl-C kills it?

Quit the program by sending a different signal to both processes, e.g. SIGQUIT with Ctrl + \.