1) It ought to, but I’ve not been supporting this project since Opera 12 was released and re-added the missing feature that caused it to be required in the first place, so I’ve not tested it.
2) All this plugin does is to put the full web page address into the title bar, which makes it possible for KeePass’s “auto-fill” functionality (which is, by default, bound to ALT-CTRL-A on Windows systems: other operating systems may vary) to work in Opera.
To test if the plugin is working, check in the title bar to see if the web address is appearing. If it is, the plugin is working and you should check your KeePass settings.
Hope that helps!