Was "this song does not have a happy ending" the original working title for la de da de da de da de day oh, or was that just a note to self about the song?
well, it really wasn't la de da then because there was no la de da part. i think the "i don't have enough time" was for something called "in the snow", which went into another chorus which was 1-time only and ended there. 
then when trying to sort things out a few years later, i probably did a couple of fixes near the beginning and then got to the 'i want to write a song' part, and just needed a more decisive, rejoiceful sort of homecoming chorus after that. when the 'la de da' solution came for that, springboarding so clearly from the 'wanting to write a song' thing, it was clear that this is what it's all about