The Nazgul. Men?

[Kuruharan]

We're given no reason to believe he is a Nazgul. Why should we assume he is?

I'm not saying that he was. I'm saying that there is equally little reason to believe that he was not. There is no way to make a real statement about it one way or the other.

The other Nazgul that are given names/titles are presented clearly as such.

Yes, sort of. But why name him at all if he was just an insignificant orc or human. I find it unlikely that an orc or human minion would be allowed to take over an assault when there were other Nazgul available. And why wouldn't the Lieutenant of Minas Morgal be a Nazgul?

I'm not trying to make a case that he was, I'm just trying to get you to acknowledge that there is about a 50/50 chance either way.