Well, this chapter is really good news to me. The thing is we know the demon clan should have to be resurrected so that we will be able to see one of the most charismatic characters of the story in full mode, and so that the future holly war will be something really great. Also we know there is a way to resurrect them. So what were the chances, in terms of plot development? It could have been released at the end of the battle between Mel and Hendricksen and I believe it was the best way to do it, or … we would have to wait for another villain like Hendricksen, to witness his development through a series or arcs, to finally reach the point in which he could release the demons, all of it in a believable way. But that was the point where we were already some chapters ago, so the perspective was a bit dark to me, since I don't like much the soap shounen in which the characters and also the readers have to endure a calvary to see the best moments of the story... The demon clan can be resurrected? Well yes! Bring it on now and lets see what happens there with them!
In other words, the chapter helps a ton the previous arc much more than any other one, since now that arc was not a waste. Lady luck is with Hendricksen and... with readers like me!!
About the details, I think there's no plot holes. We kind of knew Dreyfus and Hendricksen were under some kind of spell; we already saw how that dark demonic bird flew away from the battlefield. I thought it was mandatory to kill Elizabeth to release the demon clan, but it makes sense that only her blood is enough... So I don't find the twist far-fetched and it even makes the following arc more promising.
I still wonder if the goddesses are not behind the resurrection of the demon clan. That really would be something.