It depends entirely on the people and their ability to hold a civil debate. Without knowing more about that it's hard to say.

While your first point is certainly a reason it might go poorly, it's the entire reason they'd be debating and people can debate and not be terrible at it/towards each other.

I don't think your second point is necessarily true. Depends on the specific beliefs of all involved, but it's entirely possible that all those who have some opinion on what the purpose of a deity or multiple deities would be if they existed.

Part of your third point is 'eh' since some theists do think they've seen a god, regardless of if they actually have or not. And just because they have seen one doesn't mean they could prove one exists, so you could probably throw that part of the sentence out. I agree with the rest of it.

The last point is weird. Maybe the debate would be useless in your view, but there are more things a person could gain from the debate that might make it worthwhile and useful to them. A good amount of the userbase here probably fit in the group that could still consider the debate useful/worthwhile to them. We've all got different reasons for being here, after all. Some want to clear up misconceptions, work to change people's views, learn more from others, clear up their own things, just enjoy debating or arguing, or something else.