My issue with what they did with Asumu isn't that he didn't become an Oni, but that they jerked the audience around so much about it. Before the staff change, I was pretty sure Asumu wasn't going to become an Oni, so when the brought it up later, I was a bit irritated. I warmed to it eventually though but then it was like at the last second they changed their minds and Asumu became a doctor. There was very little buildup to that part and it gave me a lot of whiplash. Also Kirya becoming an Oni just threw salt in the wound since he was just such a little shit.

That's what I remembered, too. The recording I have of the extended Hibiki theme (basically the same theme reiterated five times) confused me, though, because it doesn't contain the clear trumpet/Ibuki theme I'd remembered. (It goes standard > vocal > guitar/Zanki > strings? huh? > partially-united, and I can't make out any electric guitar in the final version at all (yet it's got trumpet.) I'm about to go on a tear to confirm my memory of what the openings actually did. I didn't even notice that they were varying the theme (if they did so) until Ibuki showed up.

If the Riderwiki article is to be believed, it was literally that. Supposedly, there were major rewrites as the final episode was being shot.