SPOLIER:
Since I was able to figure out this movie, and appreciate it without reading "the rules"…..Granted, we're talking about a movie here...not an actual event...but I think thats what makes this movie so great. You can go off on so many tangents with it.
Exactly!! That’s why I am so intrigued by a movie like this, we each bring our own history, knowledge and emotional leanings to bear when we experience film. I feel this movie contains an illusive quality of depth, you can throw the whole onion in the pot just for the ‘flavor’ or you can peel back the layers slice/dice/roast any method you personally prefer.
I do know that every gender/age viewing this movie at my house has been compelled to varying degree to analyze it. That says something doesn’t it? To accomplish that with a cross-section of viewers.

I also can’t think of a better way to end the film
Same here somehow the ending seems ‘inevitable’ particularly after repeated viewing. But I generally am happier with a ‘Happier ending!

I prefer to think that Frank is one of Donnie's constructs.
You could also take that on another ‘tangent’ that one teenager who viewed this movie suggested. They wondered if Donnie really ‘died’ the first time the engine dropped and that the whole movie was what he felt/thought/saw/ invented in his last seconds.

Donnie is chipping away at an invisible barrier that only he can see.
I know you want to stay away from the ‘extras’ But a interesting cut out is a scene called ‘Poetry Day’ where Donnie has to read his out loud in class. The director’s voiceover states something about Donnie is coming to an understanding of exactly what is coming. Basically something like: “A storm is coming Frank says…which will destroy the children….but he will deliver them from danger…he will deliver the children back safely to their doorsteps and send the monsters back to where they came from…a storm is coming Frank says…but only I can see it (or the monsters) , because I’m…Donnie Darko”
It’s bad enough to be a teenager in the usual angst without the additional pressure of being contacted through a barrier by a 6-ft bunny quizzing your knowledge of time travel with an aura of predestination and doom about him.