"Baelor" isn't really an event episode, just an ordinary episode with an event climax. As a whole, "The Pointy End" is equally pivotal and dramatic. And as discussed above, Martin wouldn't have wanted to write "The Rains of Castamere" anyway. But whether he's actually writing the Purple Wedding or not, it's likely to come pretty early in the season.
Starting slow and building has worked for the past three years because the source material supports that structure, but it doesn't any longer. It's not like they can hold off the wedding until episode nine, and having the big turning point there is getting predictable anyway. Something
is going to have to stretch unless they're putting large amounts of Feast
in season four, and I think prolonging the trial is more likely than playing up the Tyrell/Lannister tension even further. It's hardly sidelining Tyrion to have him on trial for his life; I'm sure that, as in the books, he'll be interrupting the proceedings with witticisms whenever he pleases.