For the redirecting issue, you just need to make sure that the link to the registration page will pass along any querystring variables, for example, if the login page is named users_Registration.php, you link should look like:
<a href="users_Registration.php<?php echo ((isset($_SERVER["QUERY_STRING"])?"?".htmlentities($_SERVER["QUERY_STRING"]):""));?>">click here to registeer</a>
To automatically log the user in, you would need to modify the insert record behavior on the registration page so that it does not redirect after the record is inserted.
you would then need to apply the Security assist authenticate user behavior to the page.
Before adding it to the registration page, though, double click the authenticate user server behavior on the log in page and note each of the settings, you will want to make sure to set it up exactly the same on the registration page.
The authenticate user behavior code will most likely be added to the page before the insert record code, you will need to rearrange this so that the record is inserted first.