Modify loop but keep the original query, what am I doing wrong?

You can’t concatenate an array onto query_string like that. See Digging Into WordPress’s post about looping for the right format. Alternately, you can use array_merge() like on the query_posts Codex page.

Consider using wp_reset_query() after your loop. Sometimes you’ll also see people save the original $query_string to a new variable, modify $query_string, and then reset $query_string after the loop like so:

global $query_string;
$old_query = $query_string
// Modify your query, loop through it
$query_string = $old_query;