Just done the same trip. Daytime A380 out in F. Later 777 on return. Immigration was Ok on arrival. I didnt know about tips for mobile lounge. Still not sure I get what needs to be done there??
Had my Global Entry interview on arrival which was a breeze, so will help next time!
Lounge at IAD is a weird affair, wasnt sure whether I could just sit in Concorde Dining area and stay there or whether you ate and left. I didnt fancy anything in concorde menu so chose stuff elsewhere and then Betty told me to bring it back in as rest of lounge busy surprisingly (as flight and F were half empty). Security incidentally was empty and through in <5 mins. Not overly impressed with check in experience. Rather abrupt female after waiting for ages as only person in F queue. Boarding was a joy. Seperate F Queue. (unlike T5 , C which was a shambles boarding A380, though liked seperate airbridge!) Nice overnight flight, very young but helpful crew. Met by service manager who chatted.
Overall enjoyable experience. Would definitely go back to Washington.