The date you were traveling was the start of Spring break holidays for numerous school age children and colleges. It was also the end of Spring Break for numerous of the same. In US there usually two main weeks in March that are Spring Break. Thus you most likely hit ATL with a bunch of other travelers going through immigration and customs.

Sorry that it took so long, but immigration and customs are outside the control of the airlines. They may hire the staff helping direct people to booths, but those staff can only direct people where CBP allows.

As for your flight to LHR, as others have pointed out you were too late. International flights have additional paperwork, plus bag matching requirements that require you be on board at least T-30.

All of the rest of the details are extraneous and don't matter. Who you work for, where you are going to work, etc just don't matter.

DL may be nice and toss some miles your way, but that's about it. There really wasn't anything thato wouldve been any different on another airline.