Whilst your argument looks factual and compelling from the comfort of a warm house in safe secure 2008 I believe it is flawed.
You've taken into account facts and figure, all with the hindsight of 65years of history. What you've not done is take into account the situation, morals, imperatives and quite frankly desperation of the situation at the time.
I don't see how your grandfather’s contribution to the argument is relevant to your criticism. They obviously did their duty at the time, much the same as I would expect you would if you were conscripted and in the same position. I'm sure, much like the rest of us now, they didn't want to go to war, but they did what they did to survive, and secure their loved ones.
617 Sqn did the same thing.
At the time there was still a very real chance that Britain could lose the war and, as your examples illustrate, it's population killed, enslaved or repressed in the worst way. British cities (as well as German) had been bombed to rubble with thousands of dead civilians.
Britain and her allies at the time were fighting for their very lives and didn't have the known facts, security and the luxury of hand wringing that your post suggests we now have.
The RAF, did what it could to hit back at an enemy that had over run and enslaved most of europe in any way it could.
If they had of known of the effectiveness, IF they had of known the cost in aircrew lives, IF they had of known the allied prisoner death toll, IF IF IF...
The point is they didn't. They did the best with the tools and information they had at the time in the moral climate of the time. The Dambusters raid isn't celebrated as a military victory, it's celebrated and was publicised at the time as a celebration of the will to fight back, to damage the Nazi war industry it a way not yet done and to give the public, and forces the hope that they were not suffering and fighting in vain.
I'm afraid your cold analysis of the figures just ignores the entire setting and context for the raid. Frankly I find it narrow, disrespectful, ungrateful and clearly written as a criticism by someone who has not been in the position of spending the last 4 years fighting or wondering if you and your family would live to survive another day.
Without context bare figures are not a great guide for deciding to criticise an action or not.
I apologise if I sound rude, but the original post really annoyed me.