Being as objective as I can, I'd say your instincts are correct. It sounds very much like the kind of elaborate story our difficult child would come up with, except in her case she hates the outdoors, so it'd be some other kind of getaway, but the purported object would be the same: she needs a break, just needs to clear her head, etc. It was always something superficially plausible but then the details just didn't quite add up. When called on the discrepencies she'd just pile on more names and circumstances in hopes of confusing everyone to the point that we'd just give in, and when that didn't work, just retreat into hostility or try to pull a real "look what you made me do" stunt. The ultimate object was always the same: staving off the immediate crisis and continuing the destructive lifestyle for a little while longer.
I know how you feel. Bravo on not letting her drama spoil the birthday plans -- I know that's easier said than done. Stay strong. Pulling for you.