First off, if you don't want users to select anything, then don't use a dropdown - the whole purpose of a dropdown box is to make a choice. If you just want this field to be auto-set and not selected, then change it to a text box, make it read-only, and set its default value to the value userName(). Did you try that? If you have exact usernames in the DB, then it should match up automatically, and then your rules on that field can trigger to set the other fields automatically. Seems simple unless I'm missing something.
BTW, instead of your roundabout method for getting AD info, you could use the Qdabra ADWS tool to get the info directly and easily at a very low cost.