Good fun I thought.
Curiously “Existentialist” was a term the person in 25a completely rejected.
It seems that the term central to 9a is now almost exclusively pejorative. Something the dictionaries don’t seem to reflect.
Good puzzle, as ever from Picaroon. At first I thought it would be tough, but found it not too hard in the end.
COD 20 because of the ‘birthday celebrations’ - brings back some (painful) memories. Plenty of others to choose from though.