I still don't understand your point.
First, %_exit will always be 0 in instance X (provided there isn't an error in your REBOOT syntax). Second, the %_exit value in every other TCC instance will be whatever their last internal or external command returned (unless you have an "ON CLOSE / ON SHUTDOWN / ON LOGOFF" handler defined), and is unrelated in any way to REBOOT. There is no value of _EXIT that "correctly matches the REBOOT option".
So your first instance is closed by the REBOOT command, and all of your remaining instances are closed by Windows, and the exit codes will be random at best, and determined by whatever those shells were doing and what you might have in your TCEXIT.