Great plugin Mark! Seems like you're really on to something here.
One quick thought: for those users who previously have unchecked the option "Show Toolbar when viewing site" from their profile - activating the plugin could be quite confusing, as they'll see no toolbar on the front end of the site, and no way to toggle into edit mode.
Perhaps on plugin activation automatically setting that option to show the admin bar? I think it's safe to say if they're activating your plugin, they want to see the admin bar on the front end, at the very least give it a try. If you include a disclaimer that your plugin will auto enable the option, I think it's good call.