Yes, yes, I totally agree. I think Act 4, scenes 5 through 7 are meant to run concurrently. It’s a battle, so it’s all happening at the same time in different places. Shakespeare was one of the first (if not the first) playwrights to do that. Before Shakespeare, it was assumed that scenes run sequentially, that you couldn’t just cut to a scene that was happening at the same time somewhere else (see Aristotelian unities). I really don’t think that’s a stretch to assume that Henry’s order to execute the prisoners is a retaliation against the killing of the boys by the luggage train. It’s an interesting staging question, because it would be great to be able to see that cause and effect, that chaos that pushes Henry to make that decision. Now I’m getting ideas….
That’s how I interpreted it when I directed the play. The “alarum” was the sound of The Boy screaming. It just makes so much more sense in my mind.