I'm guessing online check-in uses different servers than reservations. Since they do this all the time, I wouldn't sweat it. It could slow down with so many people checking in at the same time, though.
While there have been times of slow website and website glitches on opening day, I think those issues have always resolved within business hours. Business hours on Thurs. May 18th should be long over before 12:01am on Fri. May 19th. I wouldn't expect you to have any problems with online check-in related to the itinerary release. That said, it's not all that uncommon for there to be issues with the DCL online check-in at midnight...sometimes it's resolved in minutes but sometimes it's a couple hours. Your chances of a glitch-free online check-in this Friday are as good as any other date.
I surprisingly didn't have any trouble booking on release day for my cruise. Never kicked me out or anything. Had much more trouble booking a DVC villa at the 7 month window. YMMV but I think you will be fine.
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