1. Reading too far into their behavior.
Not so sure about that.
Donnie Darko had a website that correlated to the movie, gave some more information. In it there were excerpts from the book Rosa wrote, some of them indicating that in the path of time travel it was possible that some of the people closest to Donnie would have some knowledge of what was going on and be working towards some goal, or memory of such after the time 'rift' was fixed, or just a solid 'feeling' when things were set right (this would correlate to the second to last scene panning across characters awake in bed)
Wish I had more time to type as I love this movie and I think there's more than one way to view it, and I think they intended it to be taken in one of a bunch of different ways. I don't think that it can simply be written off though that they act the way they do.