What if?

What if the reason for Sirius not trusting Remus wasn't as simple as it seemed? What if Severus saved James and Lily before Voldemort got to them? What if Sirius didn't get hit by the curse and fall through the veil? What if Albus never fell from the Astronomy tower? What if in order to save Severus, Albus would have to visit his long lost enemy? Rating for language.