I would agrue they were "dragged" in under a false pretense, as we know the reason for the war was not as they portrayed it. It was a power grab. Based on what we know of the giants, I think Drole's reasoning would likely be the case for most if not all the giants, as all they care about is strength. And Drole becoming one of the commandments was also due to word of his bravery being known in even the demon world, giving Zeldris a reason to make the offer in the first place.
It's true we've seen them do horrendous things, but that was in the present, after the terrible stuff during the first Holy War went down. We've strangely not shown them doing anything overtly evil in the past, nor have anything but assumptions on why the giants and fairies were against them. And Meliodas left to be with Elisabeth, I don't think we were shown him having an issue with them beyond their opposition to their relationship. I would argue that those who left were given reasons to do so, not because they dislike their clan or anything. Zeldris would be the best example - we know Zeldris' end goal, peace and being with Gelda, despite him being the commanding force behind the resurrected demons actions. He's willing to do anything to achieve his goal, and only turns against his father when it's established that there is no path to that goal.
What false pretenses are you referring to? I get that the goddesses were doing shady business behind the scenes, but that doesn't mean that the others were somehow tricked into the war. I also don't agree that Drole's reasoning would be the same as for the other giants, in fact Drole's motives don't even fully explain his allegiance. He could have just as easily tested his strength by siding with the demons to begin with, but he chose the goddesses until pretty much forced to switch sides. There was clearly something that made him consider the demons as the worse option, which later contributed to him and Gloxinia's decision to defect. They experienced both sides of the conflict and made their choice, just like Meliodas. Instead of him joining the alliance with the goddesses, he could have just invited Elizabeth to join the demons. There was opposition either way and both options would have pissed off the gods and affected the balance of the Holy War, especially if Meliodas or Elizabeth had also killed an archangel or two in the process. Just because the demons suffered in the war doesn't mean they were the victims in it. There had to have been something very wrong with them if pretty much all the decent members want little to nothing to do with the clan and if every other clan is willing to ally against them, with the sole exception being the vampires who literally feed on people.
There could be more off-screen, but not that much more given all the free space around. And Nanashi's comment about not being able to save them all would be strange if the majority had died. Plus, if there was thousands hiding underground, that's not something that could easily be hidden.
It was never shown how large the underground ruins were so it's hard to say how many people would have fit in. What we saw wasn't massively different from the people on the surface who seemed to fit into some sort of a town square in Liones, but I don't believe either one of these is really indicative of the true scale, especially when both combined would only be like 1% of the size of the giant graveyard. I thought Nanashi's comment was about him trying to heal the injured people he found in the ruins. Anyone directly hit by the DK's attack would have been vaporized, which was not something Nanashi could have done anything about.
Until Percy and gang appeared, Nasiens was the only one still living there, everyone else had fled outside. So Talisker's goal had been filled if that was what he wanted. Perhaps, or it could be the situations they were in, with their comments more focus on what they were dealing with. Seeing as they're apparently in direct communication with Arthur, and him seemingly giving out orders, I don't think they could misinterpret his orders. I think, based on what we've seen, Arthur has an overall plot, but due to the fact that his opponents are predicting the future, he's having to make things up as he goes along.
Talisker seemed satisfied with the situation after the Gorge had been made too dangerous for the fairies and giants to live in. The only thing he was doing at that point was trying to figure out what Nasiens was up to since he considered Nasiens a potential threat. It was only after Nasiens had returned the trees and animals in the Gorge back to normal that Talisker decided to unleash Ordo on everyone.
Agree, they appeared to believe him because he was a Holy Knight. That said, if ti was some truly wild claim, I doubt it would only be met with slight disbelief. Going to a neighboring village is one thing, but traveling cross country would be a task. Donny did so, but Donny is also had training. I think it would work, because only a handful of people would have had firsthand knowledge of what went down in the conclusion of the Holy War, and at Camelot, as we know the situation at Camelot was a lot more complicated then it just being wiped out. Everyone else would only know the demons were fought and the war ended. And true, Merlin didn't appear to have knowledge of those underground, though Arthur also seem unable to sense them.
The reactions of the citizens didn't appear very trusting to me. We only saw a couple of panels of them, the first one showing everyone in shock and skeptical, and the second one with some people being angry at Anne for accusing a holy knight of lying. I don't think it's possible for the events of Camelot to have remained unclear. Not only were the Sins there as direct participants who later became famous heroes for defeating the DK, but there were also the citizens of Camelot. Some were right there underground when the attack happened and the others had been rescued by the Sins earlier. They had been terrorized by the demons for an entire month before their rescue, so I genuinely can't see any scenario in which they could be convinced that it wasn't the demons that destroyed the kingdom, and it makes even less sense for them to start holding their rescuers responsible for it. The people of Liones who fought in the war obviously wouldn't believe anything but the known version of events either. I also think you're overstating the complexity of the situation. I assume you're referring to how Arthur and his knights are still around to this day, but the Everlasting Kingdom Camelot they currently live in was created through the power of Chaos, which Arthur obtained after the physical Camelot had already been destroyed. The destruction itself was very straightforward and matches Donny's account of the leader of the demons wiping out the entire kingdom with a single attack.
