I'm late to some of these discussions, but just to throw my $0.02 in...
Regarding flash photos. Maybe it's just me, but I think the plain clothes zone employees (directors? handlers?) are pretty good at the flash rules. Granted, I go in Sept before it gets crazy but they've stopped me for having the tiny "focus assist" light turned on on my camera. Always a pain in the butt to remember where that setting is
But if they're stopping me for that, I'd have to assume all the other flashes are getting yelled at. But with the number of people in each zone, there really isn't anything they can do.
Regarding the 2 park event. My first two years were where they had the event in Islands and then both parks. Both parks was a HAUL to get from park to park. Think I went two nights, was less than sober, and can barely remember in my old age but I can remember just walking for what seemed like forever through a dead area that had the Unimog truck with someone in the back of it yelling at you and a single food stand with deep friend twinkies or oreos or something. That walk sucked. And as everybody has already said, there's no good locations for houses and with the crowds these days, zones would be impossible over there.
What I think would really help with crowds, but might hurt Islands attendance, is after they close the park for the day guests, put up gates by the Lost Continent pass through gates and then another one up right at the Velocicoaster line. Allow HHN attendees starting at 8 or 9 or whatever time, to go over and do Hagrids and Velocicoaster. The rest of the park would be off limits allowing for nightly clean up/maintenance to go on as planned. Having those two rides open, plus the long walk over and back, would surely eat up crowds. Maybe have the Potter Castle light show going on or just lit up or something. If they have that halloween shop there again, maybe have that open.
I think having that 25% of the park open would eat crowds and avoid any of the issues with Suess and Marvel. If HHN attendees are able to hit the two biggest rides, at night no less, they might be less inclined to go to the park during the day so that might make it a non-starter for Universal...but having it not start until after the park's regular closing hours shouldn't affect the regular day attendees. But I think it would eat more crowds than an 11th house or an extra show and would probably cost less in terms of construction and employees.