I have a few pet peeves where manga and manga fandom is concerned. Right up there alongside bad, plot-altering translations is people that don't carefully read what's written and then complain about "strange" or "sudden" things that were already clear and even discussed by readers in previous chapters. Yes, the reason that three different people with the Hero attributes were together in that village is because that village was the home of the previous Hero's descendants. The Ancient Dragon was even confirmed to have been not a random monster that Kaiser accidentally woke up with his fight, but a general of the Demon King's army that came there specifically to eradicate the village and erase the bloodline.
Anyway, for those complaining about the reason for sending the hydra, I don't think we're really supposed to see it as the "right" thing to do. She's not exactly a "good" character. Not bad either, but not the sort that would invariably do the most gentle, merciful thing possible. With knowledge of an imminent attack specifically targeting a kingdom, the people of the kingdom were essentially already dead if they couldn't defend against a small fraction of the force. While it's not the best course of action, it'd probably have been difficult to argue since this is a critical situation where survival of a few would be impressive. Doesn't mean he thought she was right, but it makes it rather difficult to argue.
Anyway, I kind of suspect that she really did it out of a different kind of necessity: it looks like the Hero attributes don't just randomly awaken, but have to be driven out by a desperate need. Not an uncommon trope, but does provide some slight improvement over just suddenly gaining new powers. If she knew or suspected that the girls had these factors, and that the danger was severe enough that the Hero would be the only way to deal with the coming storm, then it could be a crucial situation was necessary to force the awakening. The only way to force it would be to force the girls to recognize the scale of the threat they faced so they'd know their own weakness and the potential consequences if they don't find the strength.
The biggest point now, in my opinion, is whether the story will continue to have Kaiser be this impossibly powerful monster that can kill any enemy that threatens his daughters, or it will finally face them with a situation where even their father isn't enough and they need to make use of their powers and his teachings to surpass him. If Kaiser remains the instant solution to every situation, yeah this story will seriously fail. If they finally have the girls reach him, even through the awakening of hidden powers, then it has some real potential to be more than just doting daughter fanservice.