Ah, you're right! This is the climax, and it is all falling action after this! For an English major, I was always appalling with that stuff. My mind doesn't work that way, or something.

Yes, CoS does feel like just a transition.

I agree, I don't think Lily's sacrifice made Harry invincible. It did, however, make him the first person to ever survive an Avada Kedavra curse.