How long can you stay in the parks after they close? I see people who post photos with noone in them at all at night, which leads me to believe they are in there well into the night after the park closes.
It differs by park, but security generally starts their sweep at the back of the parks about an hour after official closing time. The main gift shop in each park are open at least an hour after close. Epcot often has late diners in the restaurants 60-90 minutes after park closing time. And because they keep attraction queues open until official park closing time (anyone who gets in line before closing time will get to ride), there can easily be an hour's worth of riders still waiting in line at some of the more popular attractions.
You can hang out toward the front of the parks for 90 minutes or more most nights. Security usually won't force people out, but if there are people really lingering, at some point Security will kindly-but-firmly suggest that it's time to leave.
Back when they did the propane "blowoff" of the Illuminations Inferno Barge after all the other barges were put away, I stayed in World Showcase just to to see it a couple times (typically it was 90 minutes after closing time). I just made my way very, very slowly from Mexico over to the International Gateway bridge, taking photos of the empty pavilions along the way. Security understood what I was waiting for, and I made sure to let them know I'd be heading out to Boardwalk right afterward.
(Though the blowoff doesn't happen that way anymore. They incorporated it into the final blast of fire during the show)
As for the MK entrance, there are heavy-duty gates across the tunnels under the train station. I presume they get closed every night. So even if you hopped the turnstiles, you wouldn't get any farther.
I know there are similar gates at the Studios, and I presume Epcot and Animal Kingdom have gates, too, but I can't visualize them at the moment.
-Rob