I can report what happens with Diagon Alley, as it worked out great for me tonight. Universal Studios closed at 5 for Holloween Horror Nights. We went to Islands of Adventure at 4. Unannounced, however, they kept Diagon Alley open until 8, but you could only access it via Hogwarts Express from IoA -- HHN guests at Universal Studios could not enter Diagon. Since it was not announced, and we stumbled on it by accident, we rode the Hogwarts Express both ways with no wait, rode Escape from Gringotts with no wait, did the wand show at Ollivanders with no wait and walked around a deserted Diagon Alley in the dark for an hour triggering all of the wand special effects. What a stoke of good luck.
So, if you don't want to do HHN, head to IoA on HHN nights, and then take the express over to Diagon to have the place to yourself.