Can you purchase tickets when you arrive?

Because they're still in their preview window, I want to say no. When we were there, I didn't see anyone purchasing tickets. Once they're done with their preview open, purchasing tickets onsite, shouldn't be an issue.

I believe you might have to buy online only for now. They are not officially open yet.

They had two ticket windows open when I went.

I think they had 1 or 2 ticket windows open, but your best bet is to purchase ahead while their in the preview window.