however Miranda NG will not continue working unless they install the updates.
no, Miranda can easily run even if it didn't install updates, you need just to close the update dialog
also you can check for updates once a week, for example (tune the interval in Options - Plugin Updater)
also you can install Miranda into %APPDATA% folder, where any user has full control, or, if you are the domain admin, install Miranda on a network share and run it from there (only you will update dlls)