Oh, you don't need to do any of the certificate stuff anymore, and it wouldn't have been necessary on High Sierra anyway, that's too new! You should go ahead and delete it from keychain access so it won't mess up anything.
Try deleting Library/Preferences/widget-com.apple.widget.unitconverter.plist, then restart your computer, and see if the widget updates.
i tried Lasthenia's recommendations and it still did not work.