If I literally put a gun to your head and force you to shoot someone, you are not morally culpable of anything.
That's a situation where I think most people would sympathize with the shooter--perhaps to the extent of not feeling that any further punishment (because that was punishment enough itself) need be imposed--but I'm not sure a consensus would consider them entirely blameless. I think the judgment people would instinctively make it that situation would depend on both the shooter and the target. Two random people? Tragic. An escaped murderer (held at gunpoint) shooting a child? Add it to his charges at half price.
I see where you're going with this, but you seem to be drawing the line of choice at death, where I draw it at the shooter's finger. I really don't see why, in general, many people think that if the alternative to a choice is death, the choice isn't a choice. People make choices that they believe will incur high risk of death (even 100% in their minds) all the time.
Here's a thought. A soldier jumps on a bomb to save his friends. On the other side of the field, a soldier in the same situation doesn't. Did the one who jumped have a choice? Did the one who didn't jump have a choice? Consequences certainly seem irrelevant to whether they had a choice.
I guess I've shifted slightly from "willingly" to "had a choice". But that's because they are identical in my opinion.
This is identical to the situation with the dragon. The victim(paladin) has the same(inconsequential) choices that have no impact. Like the woman, he has the choice to resist and die, or capitulate under duress. If she is not willing, he is not willing. If he is willing, she is also willing.
I'm just going to disagree that the paladin's choices were equally inconsequential. I don't think anyone wants to get into the nitty gritty, so I'll just leave it with the idea that I would agree with you if the dragon had the paladin grasped in one claw, the NPC grasped in another, and said, "move and I eat you both, stay still and I may let you live". That isn't how I envision the mechanics of the actual situation described. But then again, I've won games that I had "already lost" because they weren't technically over and I kept trying and managed to turn it around, so perhaps we're just looking at it from different perspectives of where the line between "completely outside my power" and "real longshot" lies.
The PHB wording indicates that it's not actually a prohibition. It's a choice druids make, so breaking it has no mechanical consequences.
Not another heated philosophical discussion resurrected! Based on your preferred implementation, I don' t think we disagreed on that one though.