It seems that you have the lesson very well studied Claudia (see note on the movie "Little Children" in Claudia's post)…
You say, if I understood correctly, that the ending is frustrating because it fails to present:
1) the characters breaking up with the status quo,
2) the characters pursuing their passion,
3) any edification derived from the mantra "know yourself first".
It seems to me that you have found your way to answer the eternal question on how to pursue happiness, otherwise probably you should be so… assertive in the critic – não negue à partida uma ciência que desconhece 🙂
I think that this movie stresses that it really depends of the individuals (the characters are obviously little children when it comes to manage their feelings) but not only of themselves. And I’m not only thinking of relationships as an alternative (as you mentioned). For instance, chance plays a very important role – probably a role proportional to the childness of the adult characters… Chance is an ever present element in the plot that really gives a dim to absolve the movie from most moralistic accusation. Yes, the voice of a narrator could mislead us to think we were being lectured but for me it worked out to be a playful director deceiving the audience… Was there a moralistic approach? Showing the characters as little children as definitively some moral standard in the background, but what was it? Did you feel that the writer/director with that end was telling us how to behave? Something like "Be responsible, don’t destroy your marriage?"
Well, I didn't felt that… At most, maybe something more generalist like a saying: its hard to grow up and it's something that doesn't automatically end when you stop being a teenager! Returning to chance, it is presented with sufficient power in the plot to leave the notion that even the ending could be totally different, if just a single coincidence was removed from the plot…
In this movie, with those characters, they found out (separately) and rather lately ("Little Children"!) that not to break up with their status quo was the best solution for their problems. Did they stop pursuing happiness by doing that?
Any way… Maybe the book treats the theme a little bit more thoroughly and bluntly…