I can't definitively answer that question as I've not had mionet functioning since the day after I got the mybook. you can disable it in the web interface and then login to ssh and run top look to see if mionet is running. better yet run top first and watch what happens as you disable mionet through the web interface.
However, you can painlessly try running without mionet by completing steps 1-3 and
this will simply stop mionet without changing anything. At this point a reboot will bring it all back just as it was. I suppose if you're really afraid you can create another script named with a very high "S" number like S99killmionet something like
this would or rather should enable mionet to load during boot and then once everything else is loaded (the S# scripts execute in numerical order) it would unload mionet. Waste of compute cycles if you ask me but I can't imagine why turning off mionet is bricking your book.
when the mybook boots it reads /etc/inittab to load what it needs in order.
about 1/2 down it is told to run /etc/init.d/rcS which loads and runs in numerical order all of the S# scripts in /etc/init.d
S30network loads the network and calls network_control.sh
network_control.sh calls post_network_start.sh which in turn loads mionet.sh which itself calls the mionet daemon located in /usr/mionet/.
In theory that means that all of this is done before moving on to the next S# which in the default installation doesn't exist. If you create S99killmionet with the proper shutdown command (mionet.sh stop)it should be the last thing to run during system initialization.
During a shutdown /etc/init.d/rcK executes the kill scripts (K#*) in numerical order as well. K60network calls post_network_start.sh stop which shuts down the mionet daemon. issuing a shutdown command on an already stopped process doesn't do any harm, and likely just issues a 'program isn't running' command.
Mionet itself is not a required Linux program, and the only thing that I'm aware of which requires it is mionet itself. If it's off it just doesn't work. If the script to load it is removed or disabled it shouldn't work and the mybook should just move on to the next line.
I wish I knew why your first one bricked.