They don't need a ton of days to soft open the land. In fact, I think they don't WANT a ton of days. I think they really want to keep the experience "hidden" until grand opening. Sure they have been posting some small videos but nothing has really been shown of MUCH of the land. A few close ups of a few buildings and the very first room of the bank is all we have really been shown. The moment they let the general public in, ALL the secrets are revealed to world and I think they really want to avoid that.
This isn't Universal's first time opening a massive new land. They know the Grand Opening will bring in a massive amount of people. It is simply impossible to test how everything works under these insane conditions. No amount of soft opening's can prepare the TM's for the insane crowds. All they can do is run the rides non-stop to simulate a peak day. You don't need thousands of people riding the ride to simulate how the ride runs. It runs the same with or without people. They can let the TM's ride the ride as guests to test evac operations. Heck, I am sure they even ask certain TM's to act like crazy guests to see how the TM's deal with certain "types" of people.
I still think the reason they are holding off on the opening day announcement is to hopefully prevent it from being a madhouse. At this point, if they open the land around the rumored 18-20th of this month, most employed people won't be able to get time off with such a short notice. Now all they have to deal with is the locals that will call into work sick and the people like myself that are taking a chance hoping it will be opened based on all the best rumors out there.
My best guess is that at some point this week or next week, we will get a grand opening date for right at the end of the press event. If not at the end of the press event, a few days after, but lets face it, you don't invite TONS of press to preview a land and then turn around and invite them back a week or two later to cover the grand opening. The amount of press they have invited from AROUND THE WORLD is crazy. To expect these press members to travel basically back to back for a preview and then grand opening a week later is crazy. Sure, following this logic, they might be planning on doing this preview and then not opening the land for another month, but again, lets face it, if the land is ready for guests to enter, they are going to open it regardless of how many days it was "soft opened". Universal wants to recoup as much money as possible this summer and not opening it until the end of July would be dumb and there is no law saying that a theme park land must be in a "soft open" state for a certain amount of days. Sure the rides have to be tested for a set amount of hours, but that doesn't mean they have to be loaded with guests.