This is an interesting point. We've been assuming they would open everything simultaneously. But it would make sense to open the Hogwarts Express and the London waterfront early and open the new sections bit by bit in order to lessen crowd management problems and encourage early on-site hotel registration. Assuming the HE stations are ready by mid-May, they could soft open it to on-site crowds during early park admission beginning, say, May 19, then extend soft openings to the general public in the evenings beginning May 26, adding a few hours earlier in the day as the week progresses. In the meantime, they could go ahead and open the waterfront to the general public and begin selling "sausage-focused" foods from the cabmen's shelters.
I imagine that opening the HE, the Knight Bus, the cabmen's shelters, and perhaps adding a couple of street performers to the waterfront would be enough to encourage fans who would like a sneak peak to book onsite hotels and begin visiting the park a good month before DA is formally open. They could even open the Leaky Cauldron early if they can block off the moving brick entrance until the grand opening. After all, Harry visits London and the Leaky Cauldron before he enters Diagon Alley, so an early LC opening wouldn't be inconsistent with the narrative.
Otherwise, if they wait open everything at once, I can't help but think that crowd management is going to be significantly worse than it was for Hogsmeade, given that DA is in such a narrow, enclosed space. But this all assumes the stations are ready, and for all we know, the Hogsmeade station could be the hold-up.