http://support.microsoft.com/kb/890294/en-us <--- got all possibilitys for specifing the sharepoint lib as default template. I´d say method 2 (with group policy) - but I don´t know your company layout.
on your following questions, yes it will prevent the locking of the normal.dotm/x file.
'Cannot save normal.dotm due to file being read only' - since a normal user will not get write access, they will get this problem, but the admin got the possibility to change it and reupload it (I´m not totally sure if you´ll need an exclusive opening shift while startup to safe it)
I hope my comment was helpful - if you need further assitance feel free to ask.
regards,
Lenin
