As you've learned, it's all about taking the proper steps, firstly fly the airplane. People have been having problems like this on Piper aircraft for a long time, mainly where I live, we need the door open for cooling until time for takeoff.
On my first 135 check ride in a Twin Comanche, the examiner that gave me the ride unlatched the door just after rotation, the POH had the previously mentioned procedure to deal with the problem, but my decision was to advise the tower that I'd like to return for a door problem.
The examiner thought that I hadn't read the procedure and subsequently failed the check, I have always felt he was wrong, side slipping the aircraft and reaching over one of passengers while still in the airport traffic area didn't seem prudent to me.
Happy landings!