A couple of suggestions...
First, try to get that headers already sent error fixed first as that may be the only real issue. This error is 9 times out of 10 caused by "white space" between PHP tags that are above the HTML.
For example, this would cause the problem:
(pretend there is a bunch of code in here
Notice the space between two - so I would go through all of your includes that are above the HTML and insure that where one PHP tag ends, the next PHP tag begins immediately... you can even put the closing tag and the new opening tag on the same line next to each other to be sure, like this:
On my second suggestion, I would modify your default file. I notice you are populating it with a database field... but remember, the DFP behavior is waiting for a form trigger so it actually occurs when the page reloads and that data field may have changed... so on your update page, I would create a hidden field that first get the output from the database and then make your default file become the output of the POST variable from the form to be sure that the default file is being set correctly.