Luffy had actually fought a commander before Katakuri, he didn’t jump straight to it. It’s entirely different than Zoro, so yes, not being strong enough is a legitimate point and does apply. Also, there’s far too many characters for anyone to get a 1v1. The main character isn’t even getting one.
He can because, you know, the story isn’t over after Wano.
Because this arc is the end of the New World.
He can be pushed to extreme diff and still fight with someone else. It’s happened before.
It’s rather hilarious to me though that people think that Zoro must get a solo fight during an arc in which his Captain won’t even be getting one.