Obviously, this turned out to be true, but saying Rosa had no chance of being revealed before Luigi or Peach is kind of a fallacy.. heck if it hadn't turned true, I imagine you would have included Yoshi in there as well...
Sakurai has proven several times (wtf Toon Link?) that he's going to reveal characters when we wants to... not based on some set order that you think he should take. As another example, Pokemon Trainer was revealed months before Jigglypuff for Brawl... I just don't think you can truthfully make this kind of statement.
Actually, I can. Since both Peach and Luigi were featured in the CGI portion of Rosalina's trailer, they would have needed to have been confirmed prior to that. In every trailer after the first, the CGI cutscenes have only featured already confirmed characters and the newcomer in question. They either started working on the scenes after Luigi and Peach were confirmed or their intention was that Luigi and Peach would be revealed before they release the trailer.
There are other flaws in your argument as well. Yoshi is not a Mario character, so a Mario newcomer reveal would not be dependent on Yoshi being revealed. Toon Link was revealed to advertise Wind Waker HD, so it's not as random as you think. Your Jigglypuff argument is flawed, as she was a hidden character while Pokemon Trainer was a starter. So obviously he would be revealed before her.
I'm not saying that characters must be revealed in a specific order, but there are some obvious cases for when they should be. First off, characters on the starting roster would be revealed before unlockable. There are some exceptions, such as third party characters, but we won't know who is on the starting roster until the game is closer to release. The second is that we would see veterans revealed before newcomers except where it conflicts with point 1.
Using these two points, it's obvious that we'll see Mario, Bowser, and Peach revealed before seeing a Mario newcomer. These three are veterans and starter characters, so their reveals take priority over Rosalina. Luigi, being an unlockable for the past three games, was certainly possible to be revealed after Rosalina if he has the same status this time. However, his presence in the cutscene indicates that he was intended to be revealed before Rosalina as well.
Unless you're going to argue that Sakurai was planning on revealing all five of these Mario characters at the same time (extremely unlikely), there's no reason to think that she was ever intended for E3, hence the leaker may not have known or cared about her.