Regarding the causality discussion...
I think they start off with firm rules for themselves because Abe and Aaron admittedly have no idea how causality works. However, Aaron is more skeptical about the ill effects of causality. He says "I don't believe in that crap anyway...kill your mom before you're born or whatever...it has to work itself out somehow."
I think the key moment for Aaron is the cell phone incident. After he breaks symmetry by being careless, he begins to realize that nothing bad really came from that. He didn't die, or blow up, or disappear while playing 'Johnny B Goode.' He gets bolder. I think that is when he first gets the idea that it would be ok to start messing with his and Abe's past selves, as far as paradoxes are concerned.
So I think Jim and A-ron are correct when they suggest that causality is something that doesn't matter in "Primer." It just took the characters a little bit of trial and error to figure this out for themselves.