You could have an additional process who's job it is to create and shutdown the shared memory. This process could have some form of IPC to so the other processes can attach and detach, when the last goes, the manager process can clean up and die.

You could have the manage process fork/exec'd by the client libraries when they start so it does not have to prestart, and have setuid bit to run with the appropriate rights other than the first user to start up.

If the clients connect to the manager process with a UNIX domain socket then the manager could use poll() on all connections to monitor that the processes are alive, if they exit uncleanly the socket connection will still die.