OK, I'll just comment that even in my house, no matter how uncomfortable or inconvenienced I felt by an early arrival, I would never ask them to leave and return at the appropriate time. In fact, at my own house, this sort of thing would be much easier to handle. For me anyway. I think a large part of the discomfort stemmed from the fact that we were in a place where we were sort of making do. We had to bring everything (food, drink, serving dishes & utensils, decorations, etc.) Plus the kitchen was down the hall from the room, not right next door, so we were back and forth carrying things, and making sure everything was set up properly. It was a lot of last minute stuff.
At home, I would have most of it already prepared and set up, and just be putting out a few things at the very end. At my house, I'd be able to offer an early guest a drink and a place to sit. In this situation, it was all I could do to just say Hi and be glad that the other hostess was stepping up to give a better greeting.
Either way, I would be taken aback, especially considering I didn't know the guest at all, but I think the circumstances we were operating in just made things way more hectic.
But turning someone away and saying, "come back later"? I absolutely could not do that.