So, if you don’t know DND, to sum up, the rules and drama will sometimes be at odds with each other. 
Jophiel: Yes, exactly. First round would key off being attacked, or casting defensively which would be tough to tell if that is happening, and get an attack at someone. Second round to fourth round would be concentration check if hit with damage. And yes, rule of drama will decide if Durkon makes it instead of roll of dice.
Gray_Ghost: Yes as well. Again, artistic license and I do this a lot when describing hits. “The magic starts to take hold of you but with a scream of defiance, you push it away.” Or some such on a made save. Further, yes, it could be that Burlew added in some partial damage. Save or Die spells were seen as the bane of drama and unfair. One missed save and your character, high level or not, is dead. Further, when players roll saves more often than NPCs, statistics will get them that at some point they will fail a save. It’s why there are fewer of those in later editions. (FYI, Implosion in Pathfinder, aka 3.75, is ten points of damage per level, again save negates, and while still bad, 170 points can be survived by some characters.)
Power Attack is a feat, which just means you have to take something to be able to do it. It’s not free to everyone. Since she was a guard before a cleric, it’s not a bad assumption that she took it.
It’s still a balancing act, imo, when I’m running a game to have drama but follow the rules. I have run groups where the numbers are terrible to impossible for someone to hit or make a save and the same things for another character is trivial. (I had one character have a +5 save and another have +27 of the same save.)
My only argument about it being a chance to interrupt is that we have seen an interruption on a spell and it didn’t look like this.
And, yes, fun to talk about and I can do so all day!