It's not poor form.
It seems likely you are worrying about nothing. But why not just ask that management who previously was surprised?
- "Hey, I know I previously had not wanted to apply for this role. I've been reconsidering and would like to apply for it. I recognize I just served as an interviewer though and want to make sure it's not too awkward if I apply."
You may consider explaining too what about your life changed to make it a different situation now as this will come across as more authentic.
It's likely that if your management was surprised they probably had expected (and perhaps hoped) you were going to apply. You might find them very thankful that you are choosing to apply, especially because the position went unfilled.
Also, without knowing why the position was created, it is possible they created it specifically for you to apply. It might be a huge reassurance for you to apply (or even express interest in it).
The only caveat I can see to applying is if you were the only reason why all the other candidates were rejected. That does look somewhat self-serving. I would make sure to tell management before I applied that "Yes I did reject the other candidates but as I was honestly not interested in the position at the time of the interviews, I was genuinely trying to keep you from a bad hire. But I am interested now (and explain what changed to make you interested) and hope that my being on the earlier panel would not be counted against me if I apply.