Could my opponent play an instant card at the start of my turn, before I activate any creatures, to preemptively prevent damage from a source?
I'm pretty sure the source has to actually already be 'on the stack', meaning until the attack is done you can't play the prevent damage card.
Here is what happened in a nut shell. My opponent knew I was coming for his drider when I played feint with my assassin to tap it. Knowing this was his last chance to respond with the stack he played a damage prevention card against my umberhulk, which had yet to activate. By preventing the 30 damage I needed to kill his piece, he kept it alive for another turn.
We considered the creature the source. The fact that it had not activated did not seem to disqualify it as a source.
Here it says that a source is a signle effect from a creature, in this case the Umberhulks Confusing Gaze or Melle attack, and it implies that sources or effects last until the end of your turn because attacks on two different turns are considered two different sources if made from the same creature.