Send multiple/dynamic url parameters after login validation and registration?

You can't do this through the url... I think a work around would be to create three recordsets on the page to test for the three conditions filtered by the form post and then remove the failed redirect from the login entirely. Then you can show the messages on the page based on whether a row exists in the recordset filtered by the posted username, password, and both with the active identifier.

On the registration page you can use the Server Validation for "Unique column value" in the WebAssist Server Behaviors for form validation. If you used the wizard to create the page you can just double click on the existing server validation server behavior and edit it to make sure it has unique column validation for that field.