I anticipated these rebuttals. Firstly, Dumbledore may have had that plan of action regarding the DA ready, but simply pulled it out of the bag at the correct moment. And also, when he cried, that could have been more as a result of being touched by the situation in the office, as it was. Imagine Dumbledore sitting in his office eavesdropping on Harry’s conversation. In the context of Harry’s argument there wasn’t much to get emotional about, Harry was just angry. But in Dumbledore’s office, he was announcing his full loyalty in a much more moving manner, and that is why he cried. Or, he was faking, an elaborate ploy to get Harry to avert his eyes so he would not notice that Dumbledore was obviously not that surprised.
I agree that both of these are highly unlikely, but then again the entire premise of the theory is patchy, as I mentioned earlier.