I'm glad you liked the combination of new and old! Poor Dean, always a runner-up in his own life.

Sam learned some lessons from this, but they definitely weren't "don't fuck with your brother's head." More like "figure out how to kill the Trickster for real." It's forgivable because neither of them can function without the other. I like how you put it--each of them was selfish in giving up something (life/soul, an exit from hunting) to save the other.